"...have an unissued Lee-Enfield..." Highly unlikely. Possible, at extreme cost, but not likely. Mags came with the rifle and only one per rifle. The rifle was fed with stripper clips/chargers.
In any case, which one? A No. 4 Mk I(rear sight on the receiver)? A No. 1 Mk III(rear sight on the barrel)?
Gunparts has No. 4 mags. Definitely not new, at $41.95 each. No. 1 mags run $31.45 each(not new either). They're not interchangeable.