There's two very good options, one is
Zaubertrank at Brennerstraße 20 (St. Georg).
The owner is a Shaman who produces everything he sells himself. He is quite a character, the shop is unique and very interesting.
If you go there for the first time, bring some time, because before he sells you anything, he will (hmm, how do I put it) "introduce you to his philosophy".
It helps if you read the "Einleitung English" on his website
If you want to buy my products all are self-made - it is absolutely necessary to read and to understand this introduction. This introduction is supposed to make it easier to understand the differences between goods produced according to custom and tradition and our products. Only if we keep the conditions which are explained in the following text required by law you are allowed to buy our products. Please read our explanations sequentially as one paragraph is based on the previous one.
If you're open minded, you will like the shop, it is quite an experience.
The other one is much more "neat", it's
Destille at Hermannstraße 1 Minute from Rathausmarkt (town hall square).
I buy there a lot as it's only 1 Minute from my office. They have lovely stuff and you can try it before you buy it. I just bought Absinthe there a week ago.
The owners are very nice, always up for a chat.
Oh, and they do Whiskey tastings
In both shops you can buy different size bottles and have them filled, so you decide how much you want to spend...