Hi,
I have just bought raspberry canes online from Pomona fruits, bought 3 different summer fruiting varieties to provide fruits in succession, ave planted canes but so far quite happy with their service, good sturdy canes with lots of roots!
Ken muir online v good
Can also highly recommend Peter Nyssen online for flowers/bulbs, great quality, fantastic results, I think they're quite well priced too.
Last growing season I was a bit peeved with Thompson and Morgan, as Gnome said, buy for quality rather that quantity, but, I found them a bit stingy with the amount of seeds in the packets, eg I think the sweetcorn packet said have 36 seeds, or something like that, and it contained 22, etc. This year I'm growing seeds from everyone. Often the allotment magazines (Grow your Own, Kitchen Gardener) come with free seeds and I really learn loads from the mags. Had fantastic success last year with runner bean 'white lady' (white flowers). Thompson Morgan will drown you in junk mail though!
Plants from local market, eg leeks and cabbage-family, seriously, how many brussels does one need to grow? You just need 2 plants, more than enough for the Christmas table ;-)
Poundshop or aldi/lidl with their weekly deals for labels, twine, fleece, netting, etc.