Sega Genesis power base converter

Does anyone know where I can find a sega genesis power base converter? Are they expensive? Any specific sites that sell them? Thanks guys.
I don't think Power base converters are particularly rare - I did a search on eBay for "genesis power base" and there are currently four running auctions for this item - going for between $10 and $40 (USD). A quick google search ("genesis power base converter price") turned up a few more possible hits.