If they haven't made an effort to sell it in stores, the only place you can get it is in the restaurant. Only really large, long standing chains like Taco Bell bother to make the effort to bottle and sell their propitiatory sauces or spices, etc. because it's by all means, a completely new business they have to start.A little joint wont bother. There's some hope you can call and ask to buy a pc5 or #10 can however they ship, but that's all up to the manager of the particular franchise.
Other than that, you'll just have to come in and enjoy it on your $10 plate of wings (that nets them $3 profit) which is kinda the whole point of proprietary sauces.