Hard to say, every country has its own market gotchas. Those 50k something devs are almost always working in salaried positions.
I can count in one hand the number of technical freelancers I've ever seen or heard about but I also have never actively searched for one. On the other hand there are a lot of companies that act as middlemen between contractors and companies. These contractors are employed and paid by the middlemen, but they go on to work directly on the client's site. These middlemen make a very decent profit since they pay their employees the usual (20-40 euros/h), while they bill them out to companies for up to five times as much. In some languages these companies have a specific name but I don't know how to translate it. I think they're a bit different than the agencies in the USA where as far as I know they take a percentage of the contractor's hourly rate.
In any case I guess Linkedin is a decent starting point. In some European countries it is used by huge percentages of the working population.
Those middle men are just recruitment agencies, generally. As a standard they don't have guys on staff and will take somewhere between 15%-30% on top. The 5x as much usually have full time employees and are something different, though it does happen.
For the contractor, contracting via a respected agency means you're almost guaranteed to be paid. However, they don't really offer much value beyond that. I personally dislike dealing with them but then again, my last contract didn't pay the agent, and I still got paid.
They don't really do that much besides spam the local job sites, but being from the US you won't know the local job sites, so you can either need to invest in figuring that out or you can pay recruiters their cut.
UK wise, there are a whole bunch of sites I guess the following catch most of the population:-
60000e is about £50000. Inside London or working within the finance industry, there will be developers who'll be earning more than that figure, especially contractors. However, typically contract rates seem to be between £200 - £400 per day, with some outliers on both ends of the scale.
I can count in one hand the number of technical freelancers I've ever seen or heard about but I also have never actively searched for one. On the other hand there are a lot of companies that act as middlemen between contractors and companies. These contractors are employed and paid by the middlemen, but they go on to work directly on the client's site. These middlemen make a very decent profit since they pay their employees the usual (20-40 euros/h), while they bill them out to companies for up to five times as much. In some languages these companies have a specific name but I don't know how to translate it. I think they're a bit different than the agencies in the USA where as far as I know they take a percentage of the contractor's hourly rate.
In any case I guess Linkedin is a decent starting point. In some European countries it is used by huge percentages of the working population.