I get my seeds mainly from dealers online. Currently these are just for edibles, though I intend to add a flower section later. I have sorted these into categories:
Common Seed (Brands)
Thompson & Morgan
Exotic/Rare Seeds (Brands)
James Wong's "Homegrown Revolution" Series
Common Seed (Suppliers)
Homebase Stores - Good for stocking their own brand and other larger brands. Good range in my local store and they also have a huge range of flower seeds and bulbs.
Exotic/Rare Seeds (Suppliers)
Plant world seeds - Lots of exotic fruits
Jungle Seeds - Huge exotic range, and they also have things like ginger rhisomes
Nicky's Nursery - Huge selection of varieties, particularly of chillis and tomatoes (here for rare varieties as opposed to exotics)
James Wong's "Homegrown Revolution" Series - A small range, most of which are featured in his book of the same title
Tips & Notes
I'll pick up the odd ones from Homebase or Poundland, but in general Poundland's (And £Stretchers) are often very near the sell-by (or plant by) dates and have a much lower germination rate than specialists (which is quite expected). Homebase typically supply a lot of Suttons, Jamie Oliver and Mr Fothergill's, as well as their own brand.
I plan to harvest seeds from my successful crops to plant again or trade, but I also like the idea of purposely trying to cross-pollinate and blend some varieties together.
Other "supplies" of seeds include vegetables, nuts or fruits I've just bought in the supermarket and attempted to get them to grow into a plant. I've had success with few of these, but the ones which have succeeded are doing well.