I changed my seat belt guides over the winter and purchased them from Sopers BMW for around £17.00 each. I did a small write up on the forum "Z3 seat belt guide repair" (sorry I do not know how to get the link - Gary may be able to help here)
In all honestly it is a job!! I think my small write describes it well, if you need any further help just PM me and I will talk you through it.