I've bought powerful smithing potions from the alchemist at Whiterun. I'm not sure, but if she's only offering weaker potions, you might just have to level up a bit. The store inventories are random(ish), so if she's not offering any at all, just try later.
If you want the 25%, it is generally easier to come by. The best one you can purchase is the elixir, which does 50%. Or, you can make them yourself by leveling alchemy.
If you want the 25%, it is generally easier to come by. The best one you can purchase is the elixir, which does 50%. Or, you can make them yourself by leveling alchemy.
It spawns at random vendors. Your best bets are going to the alchemy vendors in cities, but it's possible to find them from regular merchants. I think they range from Blacksmith Potion/Draught/Philter/Elixir. The 40% aren't bad either, but go for the elixirs if you're lucky enough to find them. I think I'm usually lucky with the Hag's Cure in Markarth, and perhaps Elgrim's Elixirs in Riften. Hope this helps you.