Yes for the best feed advice ring a feed company helpline, they are very good and are the experts :)

I found Feedmark Veteranaid to be a good allround supplement for my oldie.  Cortaflex / cortavet best for arteritis x

What do you feed her at the moment? what's her weight like? and does she struggle with mobility? Feeding supplements is only going to have a decent effect if the horse's diet is correct in the first place - best option is to call a feed company and get her diet as balanced as possible, and then if you need a mobility supplement the best one I've found is cortaflex - though it's pricey!