I bought some cheap eBay spacers once and the rears actually broke off from the hub. The stress cracked and eventually broke them.
I then purchased some bonoss wheel spacers (more expensive than before, introduced by my friend)

I've had it installed for over half a year, and their quality was superior!! did not have any issues yet.
Although this is a little more expensive than before, at least the quality can be guaranteed! They also provided TUV SGS test report. As I know, it is rare for those brands to provide test report (if there is any mistake, you can comment and tell me).

I used 15mm for the front and 20mm for the rear.
Unlike most spacers could be a slight vibration from 70mph+, even at high speed, you don't feel the vibration. Here is an image of the wheels spacer