I'm experimenting with some print-on-demand products. A number of people I've talked to have recommended Spreadshirt for creating t-shirts and onesies. Spreadshirt charges a bit more than some of their rivals (a definite drawback), but have a reputation for better quality clothes and a better quality of printing. So I'm giving it a go.
My "Spreadshirt Shop" (which is still quite small while I check this process out) can be found here. Most of the items are for children, but there are one or two adult shirts as well, in organic cotton.
If you happen to give one of these items a try, let me know (honestly!) what you think of the printing quality. Or, if you've ventured into the print-on-demand realm yourself, I'd be interested to hear what your experience has been.