A Japanese Super System card is usually really cheap on eBay. Virtually free, often times, if they come in a lot with other sub-$10 games. A US version is about 10x the cost, which means the mod more than pays for itself instantly, so you are making a smart choice.
An Arcade Card Pro is also a good choice. The Arcade Card games aren't very plentiful, and most of them aren't as good as a lot of Super CD games, but the card isn't that expensive and its worth it, IMO.