Do you mean "toy" trains or "model" trains? Lionel makes models -- it seems like a nitpick, but they are two different markets really (I'm in both as a seller.)
I get mine via liquidations and sales mainly, but ours is a close-to-zero profit model designed to capitalise on a mailing list. To become an official seller of almost any of the model railroad or toy brands, you need to contat the manufacturers directly. Some sell direct and others will put you in touch with your nearest distributor.
Many general wholesale and wholesale toy companies will have stuff like the Brio/Learning Curve wooden trains, Melissa & Doug's line of trains, Whittle and other popular toy trains, but selection will tend to be spotty and sporadic.
If you are into model trains, and a DIY type, you can buy huge loads of broken, malfunctioning or cosmetically challenged model trains off of eBay. Aside from being delicate, these are not complex machines and the repair work is generally fairly simple -- and replacement parts are readily available from all the manufacturers.
Another good place to connect with suppliers is at railroading shows -- you'd be suprised at some of the deals you can put together in quantity orders. Check out RailServe's model railroading calendar Link hidden: Login to view (and bear in mind that wooden/toy trains, apparely, artwork, etc. are all just as commonly seen at shows as model railroads themselves ... so each show is like a big market.) You can also typically get even bigger bins of repairables at even better prices at the shows, plus with the benefit of being able to sort through them and see what you're getting before you commit to buy.