Might as well have a predb search https://predb.me/?search=GBA+trainer (not the best search as it will include the likes of Dog_Trainer_EUR_GBA-TRM... which does also appear to be missing from various other things).
Anyway slightly formatted if you wanted a complete list, at least according to the predb, to aim for or compare existing sources to. If you find something you reckon should be mirrored then by all means let us know.
If you care about the "joke" trainers made by lightforce and Rising back in 2006 for a bunch of kids/film tie in or otherwise old and generally uncared for games then GBAtemp is probably one of the easier places to grab them these days. Though I see gba.dellicious.de did eventually add some later in the day, even if it is missing a few like Froggers_Adventures_-_Temple_of_the_Frog_Plus5_Trainer_Multi5_EUR_GBA-RS and I am guessing Fruit_Chase_Incl_Plus5_Trainer_CRACKED_READNFO_USA_GBA-RS is missing for not wanting cracks. https://gbatemp.net/download/categories/trainers.149/?page=2