Glad to see others interest in Archie OA! I've been collecting Archie art over the last couple of years. My favorites are DeCarlo, Hartley and Goldberg, though over the past year I've come to appreciate some of the newer artists, mainly Dan Parent and Fernando Ruiz.
I empathize with your frustration in searching for older Archie art. It's a bit bewildering when you think of all the art produced for Archie comics over the years, and yet there seems to be a dearth of it available for sale. Either a lot of it was discarded, or there are are collectors who are holding onto it it pretty tightly. If I had the means and budget I would mainly collect Archie art from '71 to '77 or thereabouts -- IMO all of the artists were at the top of their game then, but then again I'm a little biased because the 70s were my childhood years and the nostalgia factor is hard to ignore. So hard to find art from that time period, apart from Goldberg and some stray Hartley, Lucey and Gus LeMoine, at least from what I've seen.
It sounds like you're hitting the main sites in your search. I've had most of my luck from eBay and Heritage, though Anthony has some good pages now and then. CAT has been a big help in looking lately.
I'm pretty happy with a lot of the art I've found so far. I don't have all of my Archie art up on CAF, but quite a few pieces/stories are here:
http://www.comicartfans.com/galleryroom.asp?gsub=168797