One option for finding a good selection of TV movies on DVD is the major chain stores--Wal Mart, Target, Best Buy, etc. Visit in person or go to their websites. Most stores' websites offer a search feature to find specific items. If shopping online, click on your selection when you find it and it will walk you through the ordering process.
Another option is to go to the specific network's website, if you know which network aired it. The broadcast networks, as well as most major cable channels that air made-for-TV movies, such as Lifetime, SciFi and A&E, now have an online store for purchasing their movies. Search for the movie you want there to see if they have it.
To find older movies, especially ones that may only be on VHS and not available on DVD, browse the online outlets that deal in used movies, such as eBay, Amazon.com, ioffer.com and Half.com. Those offer a large selection of all types of movies and may be your best bet for finding a title that is out of print.
If you still haven't found the movie you want, do a bit of legwork--most cities have at least one good used book/movie/music store or thrift stores that may have some hidden gems that are hard to find elsewhere. If it's an older TV movie (especially if it's on VHS) you can usually get them for a very reasonable price.