Hi DanielUka
I think it rather depends, each of us probably has their own network and ways to get hold of these things. I guess a lot does depend on where you are situated - e.g. there is a lot of good stuff on ebay, but that may not be much help if you are in the EU and may be subject to all kinds of import restrictions and difficulties. ebay.co.uk does not even allow antique weapons on the site, and some postal systems will not knowingly carry them (e.g. DK).
Arms fairs are really good if you can get to them, and there are also masses of auctions often catering for international buyers. Often the information supplied here is of poor quality though, so if possible its better to get to the arms fair and actually handle the items before you buy.
For me probably the best way is through network of collectors and sellers who know what kind of thing I'm interested in and will sometimes tip me the wink if they spot anything.
If all that fails, then there is also the swap forum here.
...but I guess the short answer is "anyway we can"
Chris