Welcome aboard the forum.
It's a common story, had a go with a friends one and was hooked, I know that's how I started
Even though I hate to say it, Trademe is one of the most common sources these days, but as with most things Trademe, it is at a premium.
Gun auctions are always a good source as well, I know Teds auctions in Wellington always have a few in every auction.
As for the best ones to get, that's really a personal preference, I have all x GI, as that's where my interest lies, but there is also merit in the later commercial etc ones as well, if that's your preference.
I guess the choice is yours, but i am sure you will get a lot of enjoyment from whichever you choose, but be aware they are highly addictive, and one is usually never enough
I'M a CARBINEAHOLIC!