Well various avenues...
the T-55AM2 and the BPzV was purchased from Select Military Vehicles in the UK. They come out of Peter Boyes collection... I flew to see them.
the T-54 was bought through a contact out of the UK from a man who flew to CZ to pick one out after the wall fell. Only saw pics.
the BMP-1 was bought from a collector in the states
the T-34/85 was bought from a collector in the states
the T-72M1 was bought from STV, I flew to Prague to see one, but it was not an M1, but I got to know STV. They later located an M1 and I bought unseen pics only.
the T-62 came out of Jacques Littlefields collection, I flew out to see it before finalizing the sale.
the Chieftain came from a collector in the UK. Only saw in pics.
So really a variety of ways... mostly just getting to know people and making contacts, and asking questions! The most important things is being ready to drop everything to get a piece if it is available!
As a warning, I've also been taken. RussianMilitaryTrucks took me for almost 50k... he's a UK citizen - Richard Moore. Another collector took me for $5k on a deposit, as well as a Pole for about $2k on spare parts.
Just know who you are dealing with, and ALWAYS a contract.