You can use a PCEngine SuperCD card if your system is modded, you have a converter, or do a "chopsado" mod on the card. (Or alternatively use an Arcade Card *Pro* as it also has a built-in Super CD card. Of course it's also a PCEngine card. Don't get an Arcade Card *Duo*.)
I don't know how rare the US SuperCD cards are but it's probably cheaper to use any of the above options.
The main problem with using a PCEngine SuperCD card is that the menus (for save game management) are in Japanese (AFAIR). You might want to keep your regular US CD card handy if you need to format or delete save games.