For a commercial supplier of seeds I would go to Chiltern seeds first.
http://www.chilternseeds.co.uk

You say you are not looking for out of the ordinary seeds, can you say which seeds you want. Most of the companies which supply seeds to garden centres have web sites where you can read the info and choose what you want. Make a list and then pop in to your local GC and buy without incurring postage. Some supermarkets also carry packets of seeds.

If you're not looking for anything unusual, then I'm unsure why you're looking on the internet...? I would suggest visiting your local garden centre, DIY sheds, or stores such as The Range. They all carry seed ranges from a variety of suppliers. Alternatively, if you can't get out for some reason, contact seed companies and ask them to send you catalogues.

Well, for a start, I'm looking for seeds to grow for using in cracks in paving stones:

Japanese painted fern
Erigeron
Asphodel Crane's Bill
Geranium dalmaticum
Mentha requienii
Viola sororia
Pennyroyal
Irish moss
Miniature brass buttons
Adriatic bellflower

These are some which I have listed from various gardening literature. Some of them are easy to find, but others are not.

Incidentally you can't grow ferns from seed, you'll need to collect the spores and treat them quite differently from normal seed germination. I don't know if you can buy spores but it will take about a year to get very tiny ferns abut half an inch high. I think mine are still too young to form spores but if they do I could send you some with instructions..

Thank you Steragram. My book says they are short and suitable for planting between paving. I would appreciate some spores if yours ever produce them!
Thank you also to everyone who replied. I have looked at the Chiltern Seeds site and it is very comprehensive. If I can't find all I want there, I think I shall be able to find substitutes.

This sounds like a very interesting project C'hopper. I'm not sure if you have the Latin names for the plants chosen but I think you could end up with plants which are not best suited to growing in small cracks.

Thanks for the advice Scotsgran. I took most of the list from"Which Plant Where", a book from the RHS

They probably gave the correct botanical names for the plants they recommend. I was concerned that you perhaps wanted to walk on the area after you plant it up.

Have a look at 'Paving Ideas' - choose P from the A - Z list at the foot of the page. Member Eedee shows how effective using Irish Moss was planted between paving slabs. He/she also explains how it is cared for.

We shall walk on the area after it is planted.
I looked at Eedee's paving. It's a useful guide.
We inherited a huge patio area, all flagstones, rather like a parade ground. We're trying to break it up into interesting areas. My husband has built three raised beds, and we're also going to take up some paving stones and sow direct into the ground. We shall take up other paving stones and put in different textures, such as gravel and bricks.

It will be interesting to see your journey.