Its pretty easy just to make your own. Vinegar, red pepper flakes, salt, and a bit of ketchup.
I prefer to use apple cider for the vinegar but if you want it sweeter than balsamic or red wine would do.
Really, though sweet isn't what you are going for with the sauce since the pig should be sweet enough.
Another thing I do is put a slice of extra sharp cheddar cheese on my sandwiches between the pork and slaw.