For recently released items I check Charles RO, Train World (like this website because if you click/tap on the item that is marked as 'pre-order' it will show the expected due date), Mr. Muffins Trains, Pat's Trains, and to lesser extent: Model Train Stuff, All About Toy Trains, The Train Shop, Legacy Station, Train-Station, and finally cttrains.com.
If I can't find it in any of these places or it's an item that was released a while back, I do a Google search and see where it might be found.
For any item (recently released or released a while back) I set up a Saved Search in eBay. That way - especially with the newly released items, I generally know when it becomes available.