The one that I started with was from a company that I don't see ads for anymore. Don't know if they went out of business or what. However, once I went through the stuff from that kit, I now buy the RTV mold material and the casting resin individually when I need it. I buy the stuff from Micro Mark. The main supplies you need are the RTV mold material and the casting resin.
Resin casting takes some practice, and there are tricks. I suggest researching old articles from FSM (including one of mine, but can't find issue now). Biggest secret is mold design. Think like an air bubble and figure ways you would not be trapped in mold. I suggest you be careful designing pour sprue and vent tubes. Never bring pour sprue into top part of mold cavity. Air trying to leave will collide with incoming resin. Make pour sprue J-shaped entering bottom of cavity. Provide a vent tube from every local high point in cavity. Make mold vertical with extended top so pour sprue has a considerable distance above highest point in cavity (this creates hydraulic pressure head).
If you design the mold right you can get quite decent castings without buying a vacuum pump.