Great question. It is something I have always been curious about as well, and I feel like I have been collecting for quite a while. Heck, I didn't know there were three planes until Bob's post. I do know there are two different size roll-over tanks. Had a chance to get a small one years ago, and saw a large one in someone's collection.
Back in the pre Internet days. I would find a lot of stuff at flea markets, garage sales, toy and comic shows. Although the internet did not exist I got a lot of mail order and auction catalogues. Hakes Americana was a very valuable resource. I also corresponded with a lot of collectors who for the most part were pretty willing to share their knowledge and collections. Ego has always and still does come into play. Collectors like to brag and I would just suck up the info. Early on, everything was unknown and new. A great time to be a collector!