Onions for pickling

Does anybody know where you can buy a large bag of small pickling onions so that we can pickle them ourselves

when in season all supermarkets sell them

Yes all the supermarkets have them when in season. But the growing season is different in Spain.
Normally onions for picking are in the shops around this time of the year in the U.K. I’ve spent 40+ years of doing them now so they’d be ready for Christmas.
If you are looking to do that there’s a greengrocer with a stall outside Reeves Butchers, he will have them in stock now. Think the Spanish season is late Dec from memory , which is too far too late for your Christmas pickles.
Lynn