We purchased our oil pastels and chalk pastels at Walmart and they are both the Daler Rowney "Simply . . . " brand (25 pc. set of oil pastels and 12 pc. set of chalk pastels). I think each ran us around $5 at the time? It has been awhile though . . .
We purchased Neocolor II Watercolor Crayons by Caran D’Ache (the kind you color with and then paint with water over to turn it into watercolor) and the kids have really enjoyed those and they’ve worked really well.
Another great art supply that we love are our watercolor pencils which function in the same way as the watercolor crayons. These are actually from my days of stamping/paper crafting and I purchased these from the Stampin’ Up! company years and years ago. They are a set of 24 watercolor pencils that can be used as colored pencils or can be then painted over with water and turned into watercolors on the paper. I LOVE these pencils. Unlike traditional colored pencils, they DON’T break when you sharpen them (this frustrates me to no end with normal colored pencils!).
I don’t have any advice on paints of any kind, we haven’t gotten into much with those yet. I hope this helps!