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Margie is a trusted home cook immersed in German foodways.
It is not just a matter of finding organic roses, you need to find Damascene roses (Rosa damascena). This is an early rose variety and has a strong scent, thus the carryover in taste. I would begin by asking friends who grow roses if they have the R. damascena. If that fails, then I would contact a botanical garden and see what they recommend.
I think you would be wasting your time if you used a modern, hybridized rose. They don't have the scent which is so important.
HalfPint is a trusted home cook.
Call a knowledgeable florist and ask if they can order organic roses for you. You can also try searching online.
Diana B is a trusted home cook.
I don't want to rain on your parade, but I just tried making rose water, using fragrant roses from my own garden, and got something vegetal and unpleasant. A trip to the local middle eastern market netted me a decent-sized bottle for $1.50, so I won't be wasting my time on that experiment again. Maedl may be correct that the variety of rose matters, because R. damascena aren't all that popular in the U.S. any more, whereas in the Middle East, I imagine these tough, fragrant roses are still prized.