There are many, depending on what you might be looking for, what region, etc. Jointed Rail, Trainz Forge and RRMods are among the most popular for North American stuff.
Tip: When you purchase Payware, keep email receipts in a known and good safe place. I have a separate folder in email just for receipts. Also, I suggest a log of what and when purchased and a "Backup" of the assets downloaded all in one "Payware" backup folder. All this just in case you ever re-install and or want to to a fresh install and or you just forget what and where you got your payware.
Check down in the Freeware Announcements forum as well. NARM has just released some items, and they have some very nice locos and rolling stock https://narailm.wixsite.com/narm. Also the Backshops.
Check down in the Freeware Announcements forum as well. NARM has just released some items, and they have some very nice locos and rolling stock https://narailm.wixsite.com/narm. Also the Backshops.