There are much better mustards than Grey Poupon and Stadium Mustard, though those two are not bad. It's hard to find good mustard in the US, but Trader Joe's Dijon is almost as good as it gets for a reasonable price without going to France.

Otherwise, try http://www.amazon.com/Amora-Dijon-Mustard/dp/B004AGBYRS but be warned that it is MUCH stronger than Grey Poupon and the like. Alternatively, http://www.amazon.com/Dijon-Mustard-Maille-13-4oz-Label/dp/B....