I hate to correct you Bigtom,but you are wrong when you say "99%of stuff is for sale" regardless.You have to understand that to many collectors,collecting for the enjoyment of the hobby and preservation of history as primary motives,money is of little significance.
I could point to several examples where very valuable pieces and/or collections are being donated to museums reater than being sold.A couple of examples have been posted on here.
Further,around here,I could name some collectors who have been asked to "just put a price on it" and responded "not for sale".Money can buy things from people who want money;it can't buy history from those who value it more than money.
Like many collectors I buy a few items to resell to help fund my collecting. The focus of my collection has changed over time so I sometimes sell off pieces that no longer fit in with the focus of my collection. Under the right circumstances I might even sell a portion of my globe collection, but about 1/3 of my globes and my Sinclair collection will not be for sale in my lifetime.
Back to the original topic....
The stuff is everywhere. You have to work at it! Local auctions, flea markets, automotive swap meets, petroliana swap meets, ebay, petroliana auctions, and antique malls, dealers and collectors.
At this point I mostly buy at petroliana auctions and from dealers.
Collect what you like and what you can afford. You'll likely refine the focus of your collection over time.
When I was starting out a veteran collector friend would preach to me condition, condition,condition. He would tell me no matter how much money I had to spend at a show or auction I should buy the single best item I could afford rather than buying multiple cheaper, less desirable items. That was excellent advice.
Most of us collect a variety of things but focus mainly on certain items like handy oilers, or gas pump salt and pepper shakers, or signs, or anything from a certain company. Some guys won't buy anything unless its a deal. Once you refine the focus of your collection you'll likely be less concerned about finding a "deal" than buying an item that you need to fill a void in your collection.
Go to as many petro shows and auctions as you can. Network with other collectors and dealers. The friendships you make at the shows and auctions will make the hobby that much more enjoyable.
