It would help to specify which game and/or type of mod you're looking for as there is a large wealth of knowledge on this site and people do sometimes sell items they are holding and haven't used. In general you can't really avoid import costs and they just get passed thru. Obviously group buys are a way to spread the shipping costs. IMHO there is not a single 'best place' for pin parts/mods although obviously some places have a broader range of stock than others. In my experience you tend to build up a range of sources but you'll almost inevitably find that many are outside the UK. Your question implies you've possibly not done some obvious online searching yet. So you may, for example, want to start with the Judge Dreads sticky thread under parts for sale on this site.