Evil bay or Gunbroker. Do not waste money on a reproduction Lee Enfield magazine. Only one repro works, the old Golden State Arms 5 round mag that is stamped with their name and marked "made in Japan".
Be sure you are buying a No4 mag and not a No1 mag. You can tell the difference every easily. The two stamped channels on the sides of the magazine body are different. On the No4 these channels stop short of the bottom of the mag. On the No1 mag these channels go all the way to the bottom of the mag. The two mags well fit either rifle but you well have feed issues if mis-matched.