I know it's not supporting the UK model industry but I tend to get most of my spares from Hong Kong or Japan (as do most people I know who run Tamiya as the UK importer, The Hobby Co, don't import the right quantities of the right stuff, can take ages to order stuff in when needed and it's normally quite expensive when they do).
I've used Stella Models (
http://www.stellamodels.net) a lot with no problems (and they currently have the double slipper up for $46 + postage) as well as RC Mushroom (
https://rc-mushroom.com/index.php) and RC Mart (
http://www.rcmart.com/catalog/default.php?language=en). Postage is usually very quick (7 days or so) and I've never paid any customs fees either (even when I purchased the DB-01, Slipper, Shocks, Titanium Screw Kit and a shed load of spares at the same time from Stella).
In the UK good places for Tamiya (and especially the DB-01) I've found are Brock Models (
http://www.brockmodels.com/) and Fusion Hobbies (
http://www.fusionhobbies.com/). There both on a par with each other - fast postage and more than happy to get stuff in if they haven't got it in stock (although that can take ages due to the importer - it's not their fault)
I've no experience of the GPM shocks, mine's had TRF Dampers on from build.