Worth mentioning perhaps that many Stag parts can be sourced in the US, because they are common with other Triumph models. The water pump, for example, is common with the TR7 (tho there are both 6 and 12 vane variants so be sure you get the one that matches your cover).
Easiest way to see if a particular part is available is to find the factory part number (buy a factory Spare Parts Catalog or check the on-line version at
https://www.triumphstag.net/start/technical/partslist/initialpage.php) then check the database at TRF.
For example,
https://www.triumphstag.net/start/technical/partslist/diagrams.php?diag=02-15
shows the 6-vane water pump assembly to be factory P/N 519343, which TRF has cross-referenced to GWP207 (currently on sale for 'only' $280).
I'll also mention that by far my favorite UK vendor is
LDPart even though they stock only a limited range of parts. For example, they don't have water pump assemblies but do have the parts to rebuild your assembly for about 1/5 the cost of the entire assembly.
And since some WP assemblies on the market have been found to be defective (gears too soft or cut at wrong angles, etc.); there's something to be said for rebuilding your old one.
PS, shipments under 4.4 pounds are significantly cheaper to ship from the UK, if the vendor will use Royal Mail rather than their usual shipping company.