Hi Andrew – your easiest option is to keep a look out on E-Bay for Drummond change wheels they come up regularly. They are fairy distinctive – the two pin holes being a bit of a give away, OK I know other lathes used a similar system but Drummond wheels out number those. Drummond gears are the same fitting for the early flat-bed lathes, round-bed, type B and the M type so plenty to look out for.
There were 2 different sets designated for the round beds depending on the age of the lathe. Pre 1925 lathes the full set was 20T (2 off), 24T, 28T, 32T, 36T, 40T,44T and 64T, with 25T, 35T, 45T, 50T and 63T as "extras" for metric threads. The post 1925 lathes had a different set of wheels, being 20T (2off), 25T, 26T, 30T 35T, 40T (2 off) 45T, 50T and 66T – this set will do both imperial and metric threads. It doesn't matter which set you build up really as both sets will work happily on round beds of any age.
If you haven't already done so it would be a good idea for you to join the Yahoo Drummond lathe group. Plenty of keen owners there willing to share advice, a host of photos of members lathes and a very useful set of files with information on the various Drummond lathes.
My experience in buying Drummond wheels is that generally they cost no more than those for a Myford or a Boxford – often less as they tend to be less popular and sought after, and you avoid the hassle of having to make them fit. Beware that some sellers seem to think that they are gold plated at least if not solid gold with the prices that they ask, but it's a bit like missing the bus, there'll be another one along soon so you don't have to jump in and pay silly prices if you don't want to.