You could search the RHS's Plant finder facility. If you enter pelargonium "Lord Bute" (which is lovely but not so easy to grow I found) you should see a list of stockists and the chances are that if they stock that on, then they'll probably stock other good ones. You could also get yourself a good book on pelargoniums.
Or if there are any flower shows nearby, you might be lucky with the exhibitors. They are often sold at Tatton show, but for some reason are not as popular as they were in Victorian times, even though there are some lovely new varieties.
I bought some from Cramden this year, and as Fairy says, they were excellent, my little Angel ones have already been flowering for weeks!
I love the scented ones too and have a couple of old plants that have grown quite tall and look good in the greenhouse.
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Petit Pierre, Alice, Samantha Stamp
Ballerina
(which is lovely but not so easy to grow I found) you should see a list of stockists and the chances are that if they stock that on, then they'll probably stock other good ones.
You could also get yourself a good book on pelargoniums.