If your only planning on doing a few molds you don't need a super expensive self centering vise.
I've dabbled in making a few molds and I use a grizzly vise , I think it was a bit over $100 and works just fine for me after making a set of soft jaws with a pocket and set screws to hold the blocks .
There are a couple companies out there that will make a cherry for you . They aren't cheap though , average cost is going to be $350 or so .
I choose to make my own from 01 tool steel , cut the profile on the lathe and do the flutes on the mill . Harden and temper then hone it sharp . .. going with an odd number of flutes seems to make things work better with straight flutes for some reason
I feel like I should mention , unless I want something I can't buy , it's cheaper to just buy a mold in the long term .