What are some of the materials you all use to make molds and cast parts

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You can go cheap and use RTV for making molds and epoxy for the parts to see if you want to really pursue casting. Another fairly cheap option is to buy the moldable plastic (it's a thermoplastic, multiple types available, some geared toward the home-casting community, but you can find just pellets or sheets for fairly cheap) then buy the UV 'glue pens' -- they use a UV reactive resin and come with a little UV LED for curing/setting the resin. I actually use this option (or Ave's putty or similar instead of the resin) quite a bit for small pieces. The nice thing with the thermoplastic is that it's reusable -- just reheat and reshape.

If you decide you want to pursue casting on a more professional level, you can look at things like Smooth-On (I use their stuff and really like it, but it ain't cheap).

I’m not looking to start casting and selling, but I would like to make parts for myself because it aggravating trying to find everything I want to use for my customs.

I picked up an started kit of alumilite from hobby lobby to get started

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Ah, gotcha. If you're just wanting to use it to fill in holes in your collection, then Alumilite is a good option. I'd also look at something like Instamorph for making the molds - it's the reusable plastic. You just drop it in a container of hot water for a few minutes, press your piece you want a copy of into it, let it cool, then cast. Once you're done, you can drop it back in hot water, reshape it and make another mold.