You search for societies aimed at sharing seeds. There are weird EU laws around selling seeds - any seed has to be on a list and many heritage seeds are not. Thus, people do not sell seeds, but sell access to a club through which you'll get seeds.
I did some voluntary work on an allotment, and I met some of the old geezers working on nearby plots and we'd share potatoes and tomatoes. It was great fun, and we got some great food from it.
There are quite a few forums out there that do seed swaps/parcels for established members. You can often get rare varieties for very little cost, while passing some of your own collection on for others to grow. It is probably something to try later on, but worth joining gardening forums that do it (so that you can have enough posts) to take part.
This book on tomatoes has gotten pretty good reviews: Ripe: The Search for the Perfect Tomato
There are a bunch of sites where you can get 300+ different tomotato species, and there are also some websites that list all the various seed companies from around the world.
Also, the NYT just had a favorable interview with the author of Heart Of Dankness, a new book about growers who travel to exotic countries to find new landrace strains of marijuana and then breed them to compete in the cannabis cup. That requires a much larger commitment to the craft that just growing tomatoes though.
