I am the seller of the DISA valve o-ring
The material is made of high-temp silicone (red) which can hold up to 450F and the size is perfectly fit. I own a E39 myself and I use the same o-ring on my E39.
There is no way you can get it from dealership b/c they don't have it. I believe I am the only one in the world selling this stuff.
due to large volume of order, I may made some mistake and sorry about that.
If you have any questions, feel free to contact me.