'Croquetas' (including the crab ones) are available in most tapas bars but they don't seem to be sold in supermarkets.
Seafood croquettes are popular in much of Europe. I often eat prawn croquettes when I'm in Belgium, where they're served hot. However a Portuguese bar I use here in England serves them cold. Such delights aren't generally found in the UK. The only time I've seen seafood croquettes has been when Lidl has one of their occasional special seafood promotions.
If you want a recipe, just ask good old Aunty Beeb ;-)
http://www.bbc.co.uk/...croquettes_with_56871
Chris