You can just find a sauce or recipie you like, and use the liquid ingredients listed on the sauce or in the recipie, and then substitute your rub instesd of the seasonings in the recipie.
Kind of depends what kind of sauce you want. You could just add water, or vinegar, or oil, or butter.
You can mix it with the jucies that accumulate after you are done with your dry rub, and add some more liquid, but if you do that you should bring it to a boil before using to kill off anything bad that might be in raw meat juice.
So I'm looking at a bottle of Sweet Baby Rays barbeque sauce. The liquid is
so if that's the type of thick sauce you want, you would just add your rub to those liquids.
You could do the same thing with a bottle of worcestershier,
another common one would be juice (apple maybe or orange) and vinegar.
Or mollassas to sweeten it up.
I sometimes just mix my rub with tabasco sauce and the juices from the meat.