Without question, the best source of older crayons is Ebay. I've personally been on there since 1997 so I've got 15 years of perpective to share from a collector point-of-view.
But have you noticed that in the last few years, Ebay has a remarked drop in quality and quantity? Why is that? Let's examine.
The golden era of Ebay was probably 1999 to 2006. Those first few years from 1995 to 1999 were just too few people on there to really have been considered the sweet spot for finding things.
By 2002 though, if I searched on crayons I could find thousands of auctions and of those there were dozens of vintage crayons coming up. It was actually hard and costly to even keep up.
Of course, that first wave brought out all the stuff that had never surfaced before because nothing existed to facilitate it. Selling to an antique store or through a garage sale wasn't very lucrative. Collecting was nearly impossible because the sources of finds were so spread apart. There's only so long before the majority of stuff has finally surfaced.
One of the biggest contibutors to their quality and quantity demise was the elimintation of the ability to know who was bidding and and winning auctions. In another case of a few bad people ruining something for everyone, Ebay masked the usernames on their auctions and forever changed small collecting communities. Suddenly we could no long police from shill bidding. We didn't know if there was a new collector out there. They took away an element of community and added to their element of commerce. It was as if the soul of Ebay were ripped from it.
The recent recesion didn't help either. It wiped out a good majority of the collectors. If you have no demand, items don't show up as much.
Another interested feature that disappeared recently was the ability to search on "all countries and all regions". It used to be a drop down list selection on the advanced search screen and they removed it. If you call them on it, they claim it is still there but it isn't. While it looks like it does indeed default to that, it doesn't. I just happened to put in the UK specifically and crayons were coming up that weren't in my default search. I don't happen to like having to do multiple country searches when I didn't have to before. What product removes functionality?
And despite it all, things occasionally pop up. Why just today's search revealed another Imp crayon box from England; one that with over 11 years of collecting crayons on there, I had never seen it before.
And how cool are the graphics on this? So while something truly great is far and few between these days, they still show up from time to time and it keeps me looking.