Welcome, Hunt.
In regard to kits, of course there are the standard MPC, Revell, Tamiya, etc. that you can find in hobby shops or different web stores.
For a lot of "niche" subjects, like a lot of us sci-fi fans enjoy, you can sometimes find someone who produces more limited-run resin (or multimedia) kits as well.
Starshipmodeler.com is a great source of info on sci-fi and fantasy builds. They also have an online store where you can check out some unusual or non-standard offerings.
Brandomack was right about the kitbashing and scratchbuilding, too.
For a lot more insight into those techniques, check out some of the threads in the Studio Scale modeling section.