Hi Bob, I remember seeing the TOPO guides in a few places along the way. However, I cannot be specific about which bigger towns I saw them. My comment on Conques is that it is a tourist town (in a nice way) and there are a lot of things available in such a town, and it is quite possible that you could get the Topo Guides there. Sometimes the Tourist Office sold such things and at other times you would have to get them from a bookshop. If they are unavailable there, it is not to far too Moissac where they would most likely be avaailable - and as you would be well aware, the path is very well signposted.
You don't say how long before you go - have you checked out thebookdepository, uk? If they have them available it usually only takes about 8 - 10 days for them to post out (POST FREE!) to Australia. It will take patience to find the giudes on the site, and you will need to be specific in your search but I have noticed that they regularly have them in stock. I have often ordered something on a Wednesday or Thursday and it has been here in Adelaide by the friday of the following week.
Cheers, Janet