I think that if seeds were available, then any internet search would have results full of advertisements from sellers (it's getting difficult to actually find anything useful about a plant, after scrolling past all the adverts). Most likely they are cultivars/hybrids, where seed (if produced at all) results in plants which aren't the same, as (as Lyn says) probably produced from cuttings or tissue culture, which clone identical specimens.
Surfinias are sterile, you grow them from cuttings. However, there are trailing petunias, which are quite similar, that you can grow from seed. I have tried them, some worked, some didn't. Plug plants are the easiest.
Dordogne and Norfolk. Clay in Dordogne, sandy in Norfolk.
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https://www.jparkers.co.uk/surfinia-trailing-single-flowering-collection-1012667c