If you can get an Axim (or any other PDA) with built-in wireless, that would be much simpler than getting a wireless card for your X50. You'd also be saving the CF slot for CF cards and other accessories.
I don't live in the US so I can't really advise you on where to find things cheap. I know newegg is well-priced and reputable, so you can start there. Also look up price comparison sites like
Google Product Search to get an idea of the prices out there. There's always Ebay for stuff, but I would avoid it for memory cards as it's known for lots of fakes and defective stuff.