I figured that the rear wheel are not so noticeble due to the high ET, so I need some 20mm spacers.

But I have 18" alloyes with 4x100 and I need wobble bolts.

Where can I get 20mm spacers for my Coop with extended wobble bolts for 4x100 alloys? Can't find it anywhere

You can buy them on ebay.

Personally for 20mm spacers I would use something like these - click as you attache the spcer to your hub and then use wooble bolts to attache the wheels to spacers. By doing so you don't have to buy ridiculously long woobly bolts. Whatever you do remember to buy hubcentric spacers. Also, you will most likely need spigot rings for your new alloys too.

Well I contacted a company which makes spacers and said that hubcentric spacers need to be at least 25mm, anything less is considered dangerous...
Do you know anyone who uses this kind of spacers?

I used to have 16mm hubcentric spacers for my track car no problems. You will get problems, massive problems, when only things centering wheel on the axle will be woobly bolts

PS check how thick Coupe hubs are..? Let's have a look... about 12-15mm and they got collar to help sit wheels correctly. Is it dangerous out of factory? Guess not! Now, go and tell that company to fccuk off or make you 20mm hubcentric spacers