You can buy those from alchemy shops.
If you perk your alchemy and get alchemy gear you can make stronger versions. It's up to you wether the few extra % are worth the time and perk points.
At anything below Expert you probably don't need the extreme boosts you can get from the homemade stuff, but there are still some opponents (notably Dragon Priests) who can be a pain if you're using 'stock' supplies.
its 7 perks to really get everything you want out of alchemy. And since you go through potions like water (scratch that if you drank that much water in a day you would explode) in this game the +125% to the majority of your potion creation is very nice.
I've actually found Experimenter (3) to be handy, but then if I keep alt-tabbing out to an alchemy calculator the game crashes after a while. Practically speaking you don't actually need it though, since randomly plugging in combos at the lab will eventually reveal all the effects anyway and there isn't exactly a shortage of ingredients to experiment with.