seeing this forum here and not having anything to say has just gotten the better of me...
I've decided I want to start a bit a of a cartridge collection, from .375H&H upwards, no military round though.
can anybody tell me where I can look for/order buy or buy swap cartridges?
Do people from this forum swap cartridges?

Depends where you live. If it's in the USA, then I suggest that you check out the IAA (there's a link to their website on the Links page on my website). This is the American cartridge collectors association, and they organise various meetings around the country, at which members can buy, sell or swap cartridges. They also produce a magazine for you to advertise your wants.

In Europe, ECRA does the same job (although selling cartridges across national boundaries is an extremely difficult business).

Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk
Discussion forum at: http://forums.delphiforums.com/autogun/messages/

You can find many of the large bore shells offered for sale on Auction Arms and Gun Broker (the former is by far the better site for collectible ammo). You can also tap into some of the mail order auctions through membership in the IAA, and there are some extremely exotic items which come up for sale in those events.