seeing as you're in the uk, you are probably looking at about £5 for a veiltail from a LFS or slightly more for more sought after tail types.
Ebay is quite a good place to look, good prices usually. But whats available changes regularly
honest_aquatics do great fish, all of mine have come from them (mostly from the shop rather than ebay as i live so close)
kgbettas seem to have nice fish, (and a clearance on at the moment so a bit cheaper) but not received anything from them so can't speak from experience.
Good luck getting yourself a betta and well done for asking so many questions on here first and doing all your research. Your betta will thank you for it.
Oh and pics please when you get one

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