Yes, the other way is to write to the manufacturer of the item and ask them if they have sold any to European customers, and if they have, who were these, and then follow the trail. Usually it leads to an avionics shop, and they can vary from being welcoming to reluctant because they are looking to recover the work they put into the mod. So one has to give them some money. But all one actually needs is the local CAA's file reference. It's a real shame there is no central mod database. The UK CAA has its AANs online but I know for a fact that many mods are not on there because they were done either unofficially, or in the mistaken belief that an FAA STC is good enough by itself because of the FAA-CAA treaty.
The whole thing gets a bit more complicated because there is a whole raft of treaties between various EU member states, and the FAA, for mutual certification recognition. However I think that the bottom line with these treaties is that the local CAA still issues its own covering paperwork (like the UK CAA's AAN). Whereas a few (Australia is one I know of) just accept FAA STCs directly, with no local examination.