Nothing wrong with that. It's a common chicken-and-egg situation.

Suggestions:

Now that you have some money, hire a designer or three and have them produce a couple of dozen designs for you under various themes. Now you have something to browse. And promote.

Another thought: Hold a contest of some sort. $10K to the design that pulls in the most sales over a period of time.

The best Obama t-shirt contest?

The nerdiest t-shirt contest?

The funniest t-shirt contest?

All of the above?