theres a correlation between sketch and cheap haha. mongkok area may be one of the cheapest areas, but it is quite "sketchy" (as in legit/grey imports). my suggestion is to go to wan chai, otherwise go to causeway bay or central. prices are still managable, you can always bargain.
you can check out "shop@dcfever", google it.
and a shop called "Cam2" i believe.
if you really wanna be safe, hit up joints like Broadway and Fortress