Keypart offer lower prices than Volvo do direct, dependant on engine you could use one of the base engine suppliers. Worth looking at vol-pen website as that'll have some info on as well.
bought impellor off key parts for my 2030 14.65 + vat, it in my opinion didnt look too pretty and wasnt branded, works ok
bought impellor off asap for the same but £9 something, looked pucker, was a branded johnson and will imo last longer.
V/P from volspec was £24 !!!
Not always Johnson pumps, MD17D had Jabsco. I know because I changed it to Johnson and then the gasket that comes with the "original" Volvo impeller does not fit. Not a problem though, I just buy it from Johnson instead.
If you are after actual engine spares (gaskets, pistons etc) then you will have to use Volvo, Keypart or similar if the engine is one of the Volvo origionals (MD1, 6, 7 etc). If it is parts like impellors etc then go for ASAP etc. Volvo really have tied up the actual engine spares on the older engines and charge axccordingly which is one of the reasons why I changed from an MD6A to a brand new Beta BZ482. The cost of the spares to rebuild the MD6A was £1700 just for the parts so the new engine was the better option.
I have just priced rings for my 2003 full set £200 Big end shells £100, I have been assured that for any other engine car etc Rings would be £30 per piston and full set of big end shells £30 the set. y prices were quoted by Keypart I have rounded them up but thats about what they will cost apparently Keypart are 25% less than volvo. I am about to try asap
For Piston rings try either http://www.bradfordpistonrings.co.uk/ or http://www.deves.com/ or Tipler Engineering Co.Ltd 020 7639 8761, if a 2003 is a twin cylinder then £ 200 is a total rip off!. Tipler could probably help with bearing shells as well as the originals will be either Vandervall or Glacier.
Most of the bits would be matchable by and engineering shop if placed in front of them.