I bought some pigma pens @ Joann's today, $3.29 each for .5 mm. The package they carry (at least at the one here) have tips that are too fine - .25, I think they were. Have a few of them, don't like them.
When I'm doing blocks to be signed, I iron the fabric to freezer paper, then cut into blocks. Stabilizes it enough to write on nicely, then peels right off.