Where can you still get the old classic roblox faces?

A lot of them are still in the Marketplace, just search the face name and filter to the Faces category. Some of the original ones are cheap or even free. The catch is the ones that went offsale or became limiteds. Those you can only get through resellers now, and some of the old ones cost a silly amount of Robux. If the one you want is offsale, check whether there's a dynamic head version of it, they made a few of the classics into those.