Where can you find a magazine for a colt 38 special serial 41511 Gun?

Good luck. If you do have one of the M1911s made in .38 Special, they are EXTREMELY rare... those and the S&W Model 52 were the only .38 Special automatics ever made, they were designed specifically to fire only wadcutter ammunition, and were designed for one specific bulls eye competition. Gun Broker or Auction Arms would be your best bet.