I get mine direct from the supplier in York (who also supply some pharmacies, but not Boots or H&B). Where ever you get it from, and whether you go for tablets or powder, make sure it's 100% D-mannose.
I prefer the powder as I can be very flexible with how much I take (4 teaspoons to 'load up' if I feel a tingle, down to a single teaspoon a day for a couple of weeks to make sure all is well after a bout - not that I've actually had a full-blown bout of cystitis since I discovered D-mannose).
I don't think it would have any drug interactions, as it's just a sugar, but the suppliers I get mine from are very responsive to e-mail requests for information, so if you're not sure just drop them a line (or even call them - they have an advice phone line). I can't give you a link to the website (as they sell the product), but the name will make you think of Niagara! And if you're in the UK they do same-day 1st class postage. (I don't work for them - I've just always been very impressed with their service, and the product works!).
