All good plumbers merchants; in general, single check valves tend to be full bore anyway, with doubles being restricted, although I know this isn't always the case
Travis probably won't do them, they're a builders merchants and stock a fairly limited range of plumbing fittings, you need Plumb Centre, City Plumbing or Plumbase, I'd try Plumb Centre first though as I'm almost certain they stock them, pretty sure they're made by Altecnic
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