You can find them this way:
http://www.google.de/#q=site:.de+proxy+8080&hl=de&tbo=1&output=search&source=lnt&tbs=qdr:d&sa=X&ei=xygYT-m4Gs_MtAbx2ZDTDQ&ved=0CAoQpwUoAg&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=89d4bae99cf9c97e&biw=1366&bih=638
Test with cache time 24 hours, week and so on. If you see there are pages indexed like yesterday or so then you know these are frequently updated websites so you can find fresh proxies on them. I'm not saying this is the method to find the German proxies, but on these websites might be option to sort by countries and maybe you will find German proxies.