A quick google found [link|http://www.mwave.com/mwave/skusearch.hmx?SCriteria=AA34810&CartID=done&nextloc=| this adapter] for $10.50 and [link|http://us.creative.com/products/product.asp?category=7&subcategory=40&product=266|this modem] for $50. I'm sure some searching around will get you what you need.
I had a belkin USB to RS232 adapter that 'just worked' with FC3 a while back, if that's any help.