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Are you asking about AN fuel line fittings? If so, best practice is that mating fittings should be the same material - aluminum to aluminum, stainless to stainless. Sometimes, this is not possible - I once had a stainless gascolator fitting/firewall mount that had mating aluminum AN fittings and never had a problem with corrosion or leaks.
Personally, I prefer quality aluminum fuel fittings where possible (I mostly use Earl's from ANplumbing.com). They're marginally lighter than stainless and have never had one leak!
+1 for earls, I used their products for my entire fuel system and brake lines, they go together very easy and have been leak free so far.
Summit Racing is also a good place to get AN fittings and AL.tubing , also seat belts ..BOB
I've used Jegs -- quick service (probably similar for most of the big, online speed-shops, Speedway Motors, etc...)