When all I need is cheap paint in basic colors I go to the 'oops' dept of the local hardware store. We also have building 're-use' centers nearby, where they sell recouped paint. Maybe you have sources like this nearby?
Or, you could whip up some for yourself:
1870 Milk Paint Formula
1 Quart skim milk (room temperature)
1 Once of hydrated lime by weight
(Available at building centers. Do not use quick lime, as it will react with the water and heat up. Hydrated lime has been soaked in water then dried.)
1 to 2 1/2 pounds of chalk may also be added as a filler.
Stir in enough skim milk to hydrated lime to make a cream. Add balance of skim milk. Now add sufficient amount of powder pigment to desired color and consistency (Pigment powder must be limeproof). Stir in well for a few minutes before using. For best results continue to stir throughout use. Apply milk paint with a cheap natural bristle brush. Allow project to dry sufficiently before applying next coat. Extra paint may be kept for several days in the refrigerator, until the milk sours. Double or triple the recipe for paint. Allow to dry thoroughly 3-4 hours before use. For extra protection, give paint a coat of oil finish or sealer. Color may change - test in inconspicuous area.
Very Durable Weather Resistant Paint
5 parts cottage cheese or dry curd cheese
(Dry curd cheese is cottage cheese without cream or flavoring added. You can make a similar cheese by putting cottage cheese into cheesecloth and hanging it until it stops draining. Use cottage cheese if you’re in a hurry.)
1 part hydrated lime or slacked lime
Mix hydrated lime with water to make a paste. Add to cottage cheese and stir in well. Leave for few hours - the cheese lump will dissolve completely. Select some powder pigments and add to solution (Pigment powder must be limeproof). Add more water if too thick; powdered skim milk if too thin. Strain if you want a fine paint (old nylon stocking works well).