When I was rebuilding my series 3, I used to mostly use LR Direct (
https://www.lrdirect.com/). They were competitive on price, and if you search for a particular part number, the web site was very clear which manufacturer the options were from, relative pricing and availability/lead time. I never tested their part knowledge as I was ordering particular part numbers.
I used Paddocks for a while, but if something goes wrong with an order with them, good luck getting it sorted out.
John Craddocks and Rimmer Bros are probably very useful if you actually need to talk to an expert to find the right part.
Or just use LR Workshop (
https://www.lrworkshop.com/) to hunt around the parts diagrams to find the part number so you don't need to talk to an expert. The 'Land_Rover_Series_Part_Catalogue' (or equivalent for whatever series you're working on) is very useful to show every part you need, but the LR Workshop allows you to look at part numbers across different areas of the vehicle or alternatives, which can be useful.
Oh, and don't forget ebay. That can be very useful for finding odd items (and often seems to save on postage costs if you only need one or two things, since the bigger suppliers tend to have minimum delivery costs that aren't cost effective for small/cheap items).
Good luck and have fun!
