This one is easy. Go to any store that sells paint and look in their ‘oops’ area. Sometimes you have to ask where that is. You can score really cheap paint as long as you don’t care if the tone is a little off of what you want. For example, I needed a pint of red paint today for a Christmas decoration. The one I wanted was $12 at Lowes. I checked the oops area and found red (not gloss or exact tone I wanted), for $2.50. I have gloss sealer so hey, not a big deal.
No matter what you choose remember the purpose of the project. If it’s an outdoor prop make sure that it is exterior paint.