I did put one of Michelle's designs on a ribbed tank top. It was a customized name and I cut out all of the letters and spaced them out a bit. It looks good. The shirt was actually for my nephew who will be going to DW with us and my S-I-L said that that one was her favorite.
I was able to find some regular girls tank tops at Target, but no smooth ones for boys, which is why I bought the ribbed ones. I thought about trying a cami for my DD as well.
I bought mine from jiffyshirts.com - arrived on Tuesday (ordered Sunday night). They ran a little on the big size. you can also check out shirtsupplier.com if you don't want a "brick and mortar" store.