I have had the same problem and would love to see the answer to this. They all either seem to have more than one color or have a design in the weave.

I picked up some solid black dish towels at target last night, can't remember what colors they had, but there were a few there!
Lisa

I have found some at the $Tree labeled Royal Norfolk & they also have flour sack towels for a very vintage look with one color designs.

Have you tried online at places like "Embroidery This"? They have items prepared for embroidery - including tea towels and guest towels, etc. I've also found bulk tea towels on Ebay, for about $10 for 50, plus shipping. They are "ok" although I liked the ones at Embroider This better. The ones I got on Ebay shrink quite a bit, so I pre-wash before doing any embroidery on them.

How about Linens and things and Bed Bath and Beyond. That is where I got mine to make aprons out of. They are nice and big as well.
Susan