does anybody know where to find the ingredients that the guy in the mages guild asks you for to rejoin the guild?

You can buy them in small amounts (depending on which ones you got - I'm assuming from your post it's redwort and dragons' bane or whatever) at various alchemist's shops. You CAN find them on occasion in random locations - I've found redwort here and there. Just visit all the alchemists - there's at least one in every major town.