That's a tricky question Quinn. 'good clay' really depends on your preferences, which you need to just try out and see. Some things to keep in mind are the fact that sulfur in some wax/oil based clays can inhibit silicone from curing. You should probably try some of the different oil/wax based clays since they don't dry out. Natural or water based clays need to be kept wet until you finish your mold, they shrink and crack when they dry.
research Roma Plastilina (Warning, contains sulfur) Chavant NSP (Some people hate the consistency, saying it's something like warm peanut butter. Advantage is that it's pourable.) Plast-Econ (I have just started using this, it's not really designed for 'sculptors' in mind so much as tooling and prototyping.)
Your best option to buy clay is going to be your local art supply store, if not then online is your backup.
along with half a dozen other places. Sorry I can't just give you an answer that 'product x is best and you can buy it at store z'. experiment with materials and see what works best for you. Best of luck Quinn.
The type of clay will also need to be determined by what kind of mold you're making. For example if you decided to make a latex mask you would need non hardenning modelling clay. Or anything else with undercuts that you plan on making a negative hard mold from.