Is it possible to buy this circuit?
Yes you buy the individual components.
I am searching for help on the design of a LP filter (or something else) that can smooth out a PWM-signal.
Yes but you do not have any specification to allow you to design it.
What is the acceptable ripple level?
How fast do you want the signal to move?
What PWM frequency do you want?
When it comes to the filter order i think 1st or 2nd should be adequate.
You have absolutely no foundation for saying this unless you plug in all the above numbers and see if that order of filter will do the job.