A fairly easy way in Germany is websites, for example kaufmich.com and berlinintim.de (Berlin only). There are also some in classifieds like AFAIK kleinanzeigen.de, from what I know the... less classy ones. There are also a few well-known places for street prostitution in most cities (often one is near central station) and some brothels and places for "freelance" women. I have heard that "Sex-Kinos" are the seediest indoors venues for prostitution, where you can find dubious women (maybe old or drug addicts) to give you a blowjob for like like 25€.
You rarely come across any of these if not looking for them, but if you do look for them, they aren't hard to find.