I find that sauces are usually better after they've sat for a few days. I make two different mustard sauces, one I serve cold, one I heat. It's a preference thing I guess.
A good bottled mustard sauce is the Bullseye Gold I think it's called. It's really good.
Maurice Bessingers piggy park. Hites, and shealys, are the big three store brands here. They all have awsome q too. Not to sure if ya can get online. If not pm me I was raised on mustard sauce. Hell I eat it on turky and ham also.
BTW, if you happen to be lookin for a sauce that's dead in the middle between mustard and mater ya might wanna give Pee Dee River Swamp sauce a whirl. If you eat your bbq sammiches with slaw and pickles this stuff rocks IMO!! Very different.