Potions can be bought from alchemist shops and from many others, including many Mages Guild members.
And I recommend you to just gather as many plants and mushrooms and other ingredients as you can, and just keep making potions until your Alchemist level is 25 (Apprentice). Then you'll be able to see 2 of the uses for each ingredient, and can make cure disease potions using for example Mandrake Root and Elf Cup Cap, which is fairly common.
Besides, all those 'useless' potions you make in the beginning can be sold for good prices to all general traders and alchemists.
Have a look at this thread for the Alchemy FAQ (hope it's ok that I link to other forums):
http://www.elderscrolls.com/forums/index.p...howtopic=310139