Hi Kevin
Most of the P6 suppliers will stock door rubbers - Rover-Classics, Wynns, JRW etc. Reproduction rubber is a bit varaible in quality - do a search on here, Quattro for one has some strong opinions. Personally i have bought all my rubbers from Scotts in Australia (
http://www.scottsoldautorubber.com.au/Rover P6 2000, P6B 3500.htm). All have been very good quality and worth the wait for it to arrive.
Service parts, go to your local motor factor to start with, then to Rover-Classics, Wynns etc. There has been some interesting discussion on here about suitable oil filters, should come up easily with a search. Things like spark plugs and air filters are frequently also on Ebay.
The door mirror may be a bit of a problem. They are very expensive to buy new off Ebay. I may have a spare mirror head - but it is minus glass, which is how it got to be a spare! Getting the glass in and out is a skill I have yet to master, they come out in many pieces for me! If you'd like to have a go with my spare, pm me and we can sort out sending it to you.
Chris