Amazon still have vol I listed, but only two copies of it. If they actually have any at all (and they do frequently offer books they haven't got and can't get), I doubt very much if there will be more to come as they promise.

Volumes 2, 4, 7 and 8 are still available from the publisher, but not via Mercat's website; they can be ordered from their distributors, BookSource (at the 'web' price of £26.25 per volume), on 0845 370 0067. UK postage is £2.40 for the first book and 45p for each additional book.

My guess would be that remaining stocks of vols 3, 5 and 6 (the ones I haven't got, naturally) were bought up by speculators when it looked as if Mercat might be folding a few years back, and that they will eventually appear in dealers' catalogues at vastly inflated prices.