I've copied your query from the shoppers bargains and special offers forum as it didn't seem an appropriate place.
There's no one site that caters for everything so I'm loathe to recommend a "best" site - nearly all seed suppliers sell heritage seeds (as well as more modern varieties and hybrid) - I suppose it depends on what you are looking for. A "heritage" seed is just an open pollinated variety that is grown in a specific region where it has possibly adapted to a particular growing environment - for example, there are 100s of different tomato varieties (from cherry to beefsteak and so on) but imagine if you had a shorter growing season than most so an early maturing variety would be better. In this case Alaskan Fancy is the ideal choice - however, rather than send off for seeds in the US, if you google Alaskan Fancy, you can buy the seeds from Plant World Seeds in the UK instead. A lot of online seed suppliers have a section dedicated specifically to "heritage" seeds but I wouldn't get too caught up with the heritage terminology as it is just a marketing thing.
Garden Organic Heritage Seed Library is obviously a specific catalogue which requires membership.
I'm a great fan of The Real Seeds Catalogue - it's a small seed catalogue but I've been happy with nearly all the seeds I've bought from them (the varieties are selected for their outstanding performance for growing in the UK and they do supply some rather unique varieties that you don't see elsewhere)
However, I might "shop" around and see if other seed suppliers also stock the same variety I'm after (taking into account value for money, p&p charges and personal experience of buying seeds from their catalogue and so on) Sometimes a google search is easier to pick up specific varieties available in different catalogues but if you like browsing, you could try:
Premier Seeds Direct
Franchi Seeds (Italian seed varieties)
RHS Plants
Nicky's Nursery
DT Brown Seeds
Mr Fothergills
Sow Seeds
Plants of Distinction
Thompson & Morgan
Dobies
Kings Seeds
and these are just general seed suppliers. Sites that specialise are probably better if you are looking for something in particular - for example, there are no end of sites that specialise in selling just chilli seeds (from rare to superhot)
