National Resources for Pet Owners in Need:

The Big Hearts Fund: for the diagnosis and treatment of canine and feline heart disease
Brown Dog Foundation: prescription medications
Canine Cancer Awareness
Cats In Crisis
Companion Bridge: an online community for those with animals that require significant medical care to solicit physical, emotional, and financial help
CorgiAid: Financial assistance for Corgi and Corgi-mixes
The Dog & Cat Cancer Fund
Fetch a Cure: Support and assistance for pets with cancer
The Gandalf Fund: financial assistance for working and retired assistance dogs in the event of catastrophic illness or accidental injury.
God’s Creatures Ministry Veterinary Charity
HandicappedPets: Support for handicapped and special needs pets.
Harley’s Hope: services for low-income pet owners, service animals, seniors, and pets living in short-term foster care.
Hearts United for Animals
IMOM
Joshua Louis Animal Cancer Foundation: for pets under the care of a board-certified veterinary oncologist.
Labrador Lifeline: Financial assistance for owners of Labrador Retrievers
Land of Pure Gold Foundation: financial aid for Working Dogs (Assistance, Detection, Search & Rescue, Enforcement, Military and Animal-Assisted Therapy Dogs) who have been diagnosed with cancer.
Magic Bullet Fund: cancer-specific
The Mosby Fund
The Onyx & Breezy Foundation
Paws 4 A Cure
The Perseus Foundation: support for pets diagnosed with cancer
Petkey: worldwide lost & found pet recovery database.
The Pet Fund
Pets of the Homeless: pet food and veterinary care assistance for homeless
Pit Bull Rescue Central: resource for owners and caretakers of American Pit Bull Terriers, American Staffordshire Terriers, Staffordshire Bull Terriers, and any mixes thereof.
The Riedel & Cody Fund: for pets suffering with cancer
RedRover Relief
Rose’s Fund
Shakespeare Animal Fund
Special Needs Dobermans: Financial assistance for senior and special needs Doberman Pinschers.
Tipper & Squirt Care for Cushings Fund: financial assistance for families struggling to pay medical care costs associated with Cushing’s disease
Top Dog Foundation "Bentley Grant”: for senior dogs

Illinois:
Anderson Animal Shelter: South Elgin (spay/neuter and vaccination assistance)
Animal Care League: Oak Park (assistance for veterinary care including vaccinations, microchipping and spay/neuter)
Animal Protective League: Springfield (spay/neuter assistance)
The Animal Welfare League: Chicago Ridge (pet food, spay/neuter assistance, assistance for necessary veterinary medical care)
The Anti-Cruelty Society: Chicago (assistance with veterinary care including spay/neuter)
Best Buddies Pet Pantry: Cook County (pet food, low-cost vaccinations and low-cost spay/neuter referrals)
Better Pets Clinic: Moline (spay/neuter assistance)
Blessed Bonds: Palos Park (temporary foster program)
Brown Dog Foundation: statewide (veterinary care assistance and prescription medications)
Catsnap: Champaign County (spay/neuter assistance)
DuPage County Animal Care and Control: Wheaton (spay/neuter and microchipping assistance)
Humane Society of Central Illinois: Normal (spay/neuter assistance)
Humane Society of Southern Illinois: Carbondale (spay/neuter assistance)
National Animal Welfare Society: Mokina (assistance for general veterinary care, including vaccinations and spay/neuter)
PAWS Chicago: Chicago (spay/neuter assistance)
Pets Are Like Family: Chicago (pet food pantry, spay/neuter assistance, vaccines, microchips, pet care workshops)
Pet assistance hotline: 312-725-3315
Quad City Animal Welfare Center: Milan (assistance for general veterinary care, including vaccinations and spay/neuter)
The Quincy Humane Society: Quincy (spay/neuter and vaccination assistance)
South Suburban Humane Society: Glenwood, Chicago Heights (spay/neuter assistance)
Spay Illinois: Homer Glen (low-cost spay/neuter services)
Tree House Humane Society: Chicago (pet food, spay/neuter assistance)
Winnebago County Animal Services: Rockford (spay/neuter assistance)

Missouri:
Advocates 4 Animals Pet Food Pantry: Jefferson County (pet food, spay/neuter assistance) 
Animal Protective Association of Missouri: St. Louis (Assistance with vaccinations and routine veterinary care)
Bi-State Pet Food Pantry: Greater St. Louis area (pet food assistance)
Central Missouri Humane Society: Columbia (spay/neuter assistance)
Humane Society of Missouri: St. Louis (assistance with veterinary care including spay/neuter)
Humane Society of Southeast Missouri: Camp Girardeau (spay/neuter assistance)
Northland Pet Pantry: Gladstone (pet food)
Operation SPOT: St. Louis (spay/neuter assistance)
PAWS Pet Pantry: Springfield (pet food, litter, vetting, spay/neuter services, flea/heartworm preventative, toys, treats, beds, collars/harnesses/leashes, coats, etc., for homeless, low-income, and seniors
Pound Pals Nooterville: St. Louis (spay/neuter assistance)
Republic Paw Pantry: Republic (pet food, supplies for families affected by unemployment, chronic illness, and to seniors on fixed incomes.
St. Charles County Humane Services: Cottleville (spay/neuter and heartworm preventative assistance)
Spay Neuter Assistance Program (SNAP): Christian, Dallas, Greene, Lawrence, Polk and Webster Counties (spay/neuter assistance)
Spay Neuter Kansas City: Kansas City (pet food and supplies, spay/neuter assistance)
Stray Rescue of St. Louis: St. Louis (spay/neuter assistance)