A while ago (c. 2010), I think Hartje in DE would sell frames to shops who wanted a house brand. Not sure if they still do this or what the conditions are
https://www.hartje.de/
Also in DE, there are shops that sell remainders, but you obviously don't get the selection you might need. just as an example:
https://www.radversender.de/fahrradteile/rahmen_s3
And don't ignore ebay for finding remainders either, this seller has a bunch of remainder decalthon frames
https://www.ebay.de/str/fahrradmarkt...at=78093581013