It's not exactly easy, trying to find old Garo magazine back issues. While some collectors may be trying to sell individual issues from the auction sites, the prices they're asking will be pretty high. So far, the only store that I've found that carries back issues at all reliably is Mandarake. There are three Mandarake locations in Tokyo - the Complex in Akihabara, the sprawling set of shops in the Broadway department building in Nakano, and the huge basement store in Shibuya. There are another 4 or 5 shops scattered around the rest of Japan, but they're expensive to visit from Tokyo, and the stores don't have a policy of shipping requests between cities (that I can tell).
The Akihabara shop has the biggest selection of Garo issues, but there are holes. Also, they only put out one copy of each issue on the shelves at a time. If someone else is buying Garo at the same time you are, there's going to be a fight over who gets what. The Nakano shop has maybe 1/3 of what Akihabara has, and Shibuya has maybe 20 issues total (compare that to the 12 issues x roughly 30 years that were actually published).
A couple of weeks ago, I went to each of the shops in Tokyo, and I asked the clerks if they had more copies of the issues I was missing on a back shelf somewhere, and was told "no, what we have is what's out on the floor right now" at each place. Given that the magazine's not in circulation any more, it's not like the store can just order up more copies when they want. If a specific number is out of stock, you have to wait until a collector decides to sell off their magazines to Mandarake before you can find the number you want. On the other hand, there was something I'd noticed before, and I wanted to try an experiment.
The Japanese store clerks hate having shelves that look messy. And Mandarake has a huge warehouse somewhere that holds the vast number of used books they buy back every day. So, I started buying up 3 and 4 issues every time I visited, which left large gaps on the shelf over a couple of weeks. Finally, it worked - someone somewhere got fed up with the way the shelf looked and a good 30 issues of the magazine magically appeared to plug up all the holes. I was able to get two of the issues I was missing this way. Unfortunately, I'm still missing 5 issues in the range I'm buying in right now, but with luck I may be able to find those eventually.
Why go on about this? Well, a little while ago, there was an exhibit at some museum in New York showing the first 10 years of Garo (if anyone went to that exhibit please let me know), and the guy that put it together had made the statement that it's really easy to buy Garo issues - just place an order online through the Mandarake website. The problem with that claim is that doing a search on Garo doesn't bring up many hits, and you're buying the magazine from a specific store, not from the warehouse. If a store only has one copy of a specific issue on the shelves, you're in competition with people like me for it. So, bottom line is that it's not all that easy to get Garo after all. If you really like a specific Garo artist, then contact Mandarake and buy compilation volumes of that artist's work directly.