If you don't mind buying a "fake" pro duo, Dealextreme has some ridiculously cheap pro duo cards (like $16 for a 4gb card). They work fine in the PSP, though there are some minor limitations (read the product reviews on Dealextreme for details).
Also, you can get a MicroSD-to-MS Pro Duo adapter. So instead of buying pro duo cards, you can buy microsd cards and use them in the PSP. Again, there are some limitations, but they work great for most applications. Dealextreme sells these adapters as well.