rebuilt calipers are easy to source, and many large cities will have wholesale brake rebuilders that can do the job for you, too. Nothing special about Alfa calipers, in fact they're very generic ATE parts, with variants used on VW, Porsche, BMW, Volvo, etc. The cylinders are part of the caliper and are not separate pieces, so I'm guessing you want a set of pistons. They're available, too, but rebuilt caalipers are both cheaper and easier to source, so why not go that route. For all four rebuilt calipers, you're looking at ~$500, but I've used a local rebuilder to have mine done for ~$45 each on a few of my Spiders. Shop around before letting your mechanic do more work than is required.