If it is only the insert that you are looking for, and you already have the .303 magazine shell with the plate rivetted in to take the insert, then the little 5 round magazine was taken from the standard BSA Sportsman .22RF rifle and modified, by dint of re-profiling the side-plates with a small cut-out each side, and inverting the release latch so that it was at the top of the magazine instead of the bottom. This simple alteration provided BSA, who manufactured the Britishicon No.7 rifle, with an economical answer to the supply of just 2,500 mag-inserts for the R.A.F. contract. The reversal of the latch permitted insertion and removal from above, rather than below as in the sporter.
The only problem is that you may have to purchase a complete BSA Sportsman to get the magazine; but a well-used example is comparatively cheap when you consider the intrinsic value of a complete and operative No.7 rifle. If you are lucky, you may find one of these magazines in a trader's box of bits at an arms fair.

Sadly, yes, the original special outer .303 magazine shells are hen's teeth, and, if you can find one, may command as much money as an entire Lee-Enfield No.4 rifle.

To see more of the rifle and magazine just type "Lee-Enfield Rifle No.7" into Google. When you eventually get to the bottom of the page you will find detail of the insert.

Hope that helps.