The RS485 is a transmission hardware standard, as is RS232. If you are handy with building with ic's you can make one cheap, as the conversion IC's are plentifyll, Maxim should have, but at that ebay price it may not be worth going that route.
If you do a Google for RS485, there should be plenty of explanation sites.
B&B specialize in those products, they can be pricey, but they offer things like RS232 conversion for Allen-Bradly RS485 PLC's which are way lower than the OEM price.
Keep in mind that the RS485 is capable of much higher transmission rates, so if you convert, you have to be able to communicate at the lower RS232 rate.