Ivan,
Yes P226 15, 17 and 20 rounds magazines will function in a P228. SIG Sauer and Mecgar (extremely reliable after market manufacturer) preban (for civilians) 15 rounds magazines are still available, check Shotgun news. I haven't seen preban 20 rounds SIG Sauer factory magazines in a while but I'm sure they're around. Preban Mecgar 15 rounds magazines for the P228 are still about. I've heard of trouble with these magazines but I cannot confirm it first hand. Using P226 magazines for legal CCW carry defeats the purpose of the P228's shortened grip.
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So many pistols, so little money.