American Models has their products listing on their web site, but they will mail you a printed catalog for $10. Generally, if you have bought a product directly from them recently, they will mail you a free copy of their catalog when it is updated.
B.T.S. has their catalog available online, which you can order in printed format for $8. It lists all their kits and their detailing parts in all the scales they carry.
All of Lionel's catalogs are available from this page. Note that they have a separate one available for their American Flyer products.
MTH's newest catalog is available online, or you can go to the page to find out how to receive a printed version. MTH is slated to be closed in May 2021.
For those who enjoy collecting older A.C. Gilbert A.F. catalogs, they are available in print versions from various sources, and RFGCO sells PDF versions.