Well, it says in the original parts list for the No5:

PIN, taper, No.0000, .108-in. x .75-in.

Assuming that the pins in a no5 are not "Enfield Specials", like all but one screws in a SMLE, then the "American Standard" is probably a good place to start.

Commercial class taper pins SHOULD all have the same rate of taper; i.e., 0.250" (+/- 0.006") per foot.

The 0000 size is smaller than the "standard" range ( 0 to 10), but the rate of taper is still the same.

If your local Industrial supplier doesn't have them, try some of the places that serve the miniature engineering fraternity. It's surprising what goodies turn up in places haunted by blokes who build miniature steam engines. If they have the pins, they will very likely have the reamers.

Anyone familiar with building "aftermarket" barrels for AR-15s will have had some need to use tapered pins to retain the sight post/gas block. However, I have also seen these items successfully retained with two heavy grade roll pins per hole, a short pin nested inside the other, to follow the taper. "Proper" pins are a better look on a No5.