When I rebuilt my 3.0L, I basically got the part numbers and just shopped prices on Google. Realistically, there are two part numbers -- the Mercruiser part number that is official, and the Sierra one which is one of the major part suppliers that try to replace Mercruiser. For example, in my '93 engine, the Mercruiser part number for the Oil Pressure Sender is 815425T. The Sierra part number is 18-5899. So, with both part numbers, I was able to Google a price just by searching on the part number, and as long as there was a warranty, I took the best price (including things like shipping costs and time, etc).
You can look up Mercruiser part numbers for your 3.0L here:
http://www.mercruiserparts.com/selectDocs.asp?doc_nbr=816323+90 and once you find the Mercruiser number, you can do a Google search to find the Sierra number, then keep Googling.
My major parts suppliers were:
http://www.mercruiserparts.com
http://ebasicpower.com/index.php
http://www.masonmarine.net/
http://www.marinepartsexpress.com/
And, of course, eBay and iBoats right here.
If you can find the part number, Google is your friend.