There are no alcoholic beverages made from the "nectar" of flowers per se. It would be very difficult/expensive to collect 100% flower nectar straight from flowers in sufficient quantities to make a beverage. Having said that, honey is essentially flower nectar collected by bees and regurgitated in the hive as a future food source. You can enjoy fermented Honey as Mead in a wide variety of flavors and flower varieties. Bees store the nectar in a "honey stomach" so there is no digestion of the nectar. Read in more detail here:
"Although many sources refer to the honey bee crop as the 'honey stomach,' it is not a place where consumed foods are being digested in honey bees."
In their book, Honey Bee Biology and Beekeeping, authors Dewey Caron and Lawrence John "Larry" Connor define the honey stomach as a a "honey sac."
It's "an enlargement of the posterior end of the esophagus in the bee abdomen in which the bee carries the nectar from flower to hive."
Bee vomit? No way. It's where nectar is stored. It's not a stomach as we know it.
As mentioned before, beer is made with a flower called hops. There are many other beers and meads flavored with dried flowers.
As for where to buy Mead, you can most likely find it any decent beer/wine/liquor store and easily by searching the web.