UK: Where to find 15mg Niacin?

I am genuinely asking for a friend, who was advised by his GP to take Niacin at a reasonable level to help with his diabetes. I'm not diabetic myself, I just know that he said he couldn't find anything that was below 125% daily intake and from what we've read high doses can cause liver damage and spikes in blood sugar.

Does anyone here have any recommendations? Or know where I could ask for him? I'm a little stumped myself, so thought I would ask. Thanks! :)