Odds on a large hardy white water lily like that is either: Alba, Gladstonia, or Richardsonii
Varieties like that are very easy to get by mail order, indeed, one of the days I'll be listing alba and richardsonii on e-bay when next I get to thin those out here.
I wouldn't be surprised to find them readily available in search engine under 'water lily richardsonii' 'water lily nymphaea alba 'water lily gladstonia' those three searches should rustle up some pages with photos for you to compare with what you have seen
Alba is nowhere near as attractive as the other two, richardsonii, is, well, spectacular.... gentle rippling lily pad edges, prehistoric dragon like juvenile leaves, gob smacking flowers maybe 10" wide, a gorgeous plant for big fish to loaf among. Richardsonii rhisomes grow at the rate of about two feet a year and probably produce two to three times as many (better) buds than alba
A lot easier than taking pot luck with what's local is finding named varieties on the web, take care to check out the seller has a good range of plants, photographs of them in flower and reacts to e-mail enquiries promptly. A lot of sellers on e-bay are dedicated water gardeners, their descriptions should have accurate names and plenty of comprehendible information about cultivation of the variety
Alba's get dumped on the market quite cheap, reason being Walmart's sells a ton of them to folk with silly little ponds mislabeled as 'choice' waterlilies, buyers soon realise alba is quite invasive and large, what few buds it makes is often smothered under a huge dome of foliage that likes to be 12' wide... and several feet high, lol
regards, andy
