hello! I'm from Poland. Since 2008 i have my lovely car

Black pearl audi 100 quattro sport with 2.3 NF engine. I need some help, because i must change REAR SHOCK ABSORBERS. This model has original -20mm suspenssion low. But here i have much problems to find original absorbers. I can buy only typical absorbers to audi 100 with typical suspenssion (not -20mm).
What would i do? What kind of absorbers can be put to this car? I dont want put normally absorber without -20mm spring.
Maybe some kind of 'billstein' can be good? Please help me
thanks!
Pawel