I found a tamarind that was mostly powder, how do I avoid buying these?

It sounds like you’re describing mold spores, which I’ve occasionally seen in tamarind. I don’t know of any way to check for this, short of cutting them open to check, but tamarind isn’t particularly expensive so you might just buy a bit more than you expect to need. Often people buy tamarind extract instead, which of course isn’t subject to this problem. (Except for beverages, you really don’t lose anything by using the extract, IMO.)