Instead of buying expensive buckets of chroma key paint you can have a hardware store mix up a bucket by giving them the correct PANTONE code for the color you want. Just give them PANTONE 2735 for a blue screen or PANTONE 354 for a green screen. You could also find similar colored fabric from Wal Mart and use that instead of actual setups.

And remember to light your wall separately from your subject, and light it well!