Hi James and welcome to GoY. I have never heard of 'grafting' tomatoes and can't see how it could work as they are basically an annual plant in the UK. You can grow your tomatoes from seed and this will be available in a month or so in the garden centres or you can go on line and fine a specialist tomato seed grower if there is a particular variety you are looking for.

Yes, Mg, there is such a thing as grafting tomatoes (I looked it up on the web). This is usually a commercial proposition due to the amount of work involved, the benefit being a virus free rootstock and a more even and prolific crop. grafted tomatoes are available from some garden centers, Dobies being quoted, but they are about three times the price of normal ones. Personally, it is not something that would interest me, six plants from the local nursery, but I suppose that it could be a fun project for some.
Sorry, I have no idea where to get the initial seeds from. Why not try a horticultural/agricultural seed supplier's web site rather than one for garden seeds.

Oh well live and learn... don't think it makes sense unless you are a huge commercial grower.

Beaufort and Maxifort tomato seed seem to be the main varieties used as rootstock. An internet search might do the trick, but I think that bulk purchases might exclude the amature grower. Although you will get a far greater crop, professional information suggests that as far as resistance is concerned then the scion variety needs to match the rootstock. I'm afraid, unless you are going into production, then I would stick to growing half a dozen conventional plants of your favourite variety.

Totally agree Jimmy having checked this out...