Niche companies vary a great deal in their approach, some being natural only, many using more naturals than are typical in mass-market fragrances (including me) and some are not really niche at all but offshoots of mainstream brands cashing in on the cachet.
I've given advice on the synthetics I think are most useful for someone starting out on my blog, and I've also recommended some equipment you may want to consider buying there too.
If you want to buy synthetics for use in fragrance creation I have a, slightly random, collection myself that I can sell on (and I'm happy to help source anything you're struggling with if I can) - email me at [email protected] if you'd like a list.
There is a wider selection available from De Hekserij in the Netherlands, who ship to the UK and as they are in the EU there are no customs complications or charges. Olfactik are in the UK and stock a smaller range of synthetics (some are a bit pricey IMO but no issues with quality) and recently Hermitage have started to stock some synthetics alongside their long-standing and very extensive range of naturals.
If all those fail then Perfumer's Apprentice in the US stock a wide range and ship to the UK, but you need to be prepared for customs delays and to pay VAT and potentially other Duty on the package before it's released to you.
Between those you can get hold of most things. As I've recommended elsewhere I don't think it's good to use pre-prepared bases unless the supplier is prepared to tell you what's in it or it's an industry standard base you can be confident will be good, consistent and continue to be available.