Large chain of craft stores, like Hobby Lobby. They usually have almost an entire side of an isle of acrylic paints at different prices (from 89-99 cents up to $2-$3 or so), many different colors and several brands. I usually get the cheaper ones. They worked very well on the plastic buildings I have painted, which are mostly Ameri-Towne. I have never tried it, but you might also be able to order on-line from them? I usually go to their stores, there are several in my area.
I have used the old Testors brands that were discontinued and didn't notice a lot of difference between those and the less expensive craft store paints? However, I don't do any air brushing, only paint by hand with a brush and I'm not an expert modeler like some here are. I have not tried any of the new Testors replacements for their discontinued brands, but those should also be available from hobby stores and possibly some craft stores.