Aftermarket parts...

warwheels lists all the AM for just about every miltary wheeled vehicle, armored cars, IFVs, trucks, etc.

Once upon a time, Great Models used to have related items if you happened to be looking at a kit, for instance a Tamiya Tiger, it would list related items. That feature was a little iffy because it would often list after market items of different scales but the same or similar subjects.

Some kit suppliers have references in their online catalog that shows available aftermarket stuff for many of the kits they sell. I know Squadron and Roll Models online catalogs have this feature- I assume many others do.

I am not sure if there is a database for everyone. But if you search a site such as Hannants, there is an easier way to find any parts. A lot of on line stores will do similar so it depends which you use. If you put in the scale and the type your after, Me 109 for example, it will give you all the options. You just have to work out which ones go with your kit.

Only problem there is of course no online store stocks everything. But they do tend to stock the main suppliers. But there's often other items out there from smaller companies, and I have usually come across those from places like this.

If there is a database out there, it would be pretty huge, but might be handy.