I have two suggestions……
First, resist buying pots simply because they’re inexpensive or ‘a great deal’. The pot is more than a container, or we’d be sticking trees in tin cans and plastic carry-out containers. The pot is a work of art in itself, and if you’re patient you will find—and recognize—pots that are aesthetically attractive. I’m talking about balance, proportion, walls that aren’t clunky or heavy, really nice glazes, etc. There are many makers who do a sloppy job of glazing and describe the resulting blobs, blotches, and asymmetrical smears of color as ‘artistic’.
Second, David de Groot’s book, ‘Principles of Bonsai Design’, has excellent information about matching trees and pots based on visual proportion. I haven’t seen it yet, but his revised second edition just came out. It might be easier to find than the first edition at this point.