I too collect cartridges of all kinds ,nothing specific ,anything I can get my hands on......new,old,obsolete ,wildcats etc. I usually try to get at least 3 examples of each, as one gets the headstamp cut off for display, one full round and the bullet/bullets and sometimes the empty brass ,this gives a good reference of the type of cartridge being displayed.Of course I would never cut off headstamps from rare examples, probably get tarred and feathered for that.
I used to get deact. cartridges from Ammo One in the US about 6 years ago ,but he stopped shipping to Canada (not sure how he got away with it in the first place ?) but he had/has some good stuff.
Apoc69 (forum member) has a list of a few things on the "reloading/ammo EE ,we exchange stuff all the time (lives close by, very nice fellow)
As mentioned ,gun shows are a good bet, I do that all the time, just need to know what you need or want (bring a list)and take your time.Some vendors just have a big bin full of everything (usually common stuff ,but if you need just a few bullets to complete a full round, and you have a means to pull them, that`s helpful,and usually get a good price for volume) Other vendors will have more specific cartridges for sale ,intended for collectors.......bring a fat wallet.
Sometimes I`ll see brass for sale on the EE (small quantities) and purchase those and slap in my own caliber specific bullets in them, the remainder I can sometimes sell or swap at gun shows.
There are some really nice ,helpful folks here on CGN that have helped me out in my quest ,and keep in touch with them in case they come up with something new that I can use in my collection.I always pay a finders fee and pay what they need for their supplies and time and trouble......I don`t ask for free stuff ,even if offered.
If I ever complete my collection (within reason...need some big bore stuff) I will display my collection in my gun room ,in a large shadow box frame (I used to be a picture framer)........gonna be a HUGE frame...LOL
Good luck and happy collecting.
Paul T
****always looking for different cartridges*****