Ah, I love dragging people into modern German glass if I have half a chance...
I started out at Kringloop shops and flea markets, but I got too frustrated with the lack of good glass on offer and I'm now a second tier collector. I have bought most of these at Marktplaats in the past year (only collecting a year myself), mostly from sellers who themselves go to the flea markets in Germany and/or Belgium. The prices on Marktplaats are higher than at a flea market, but there is more and nicer things to be had. Shipping costs rather add up, sadly, but I've resigned myself to that sort of spending.
Some vases I bought at Ebay Germany. I think the prices for handmade German glass from the 50s - 70s (my collection period) are quite reasonable (compared to Czech, Scandinavian or Dutch), and especially with German glass there are chances for bargains from 1 euro auctions.
I have references and book suggestions. But maybe you should just email me personally (just click on the link on the left) before we completely derail this thread and the mods will seriously start to frown on us

Astrid