looking for all guild stores inventory
 

Is there a way to find what guild store has a particular item besides going to every location in Tamriel?

No, the data is only accessible as you open the guild stores and search within the stores.
Scanned data of some guild stores (but I doubt it tells you where the data comes from, which trader at what location) can be found at the Tamriel Trade Center Website: https://tamrieltradecentre.com

It does report which trader, at what location, and what guild. Not whom, in the guild, posted the item however. That makes TTC a good way to find inexpensive items: hit the cheapest few traders listed for an item, and you will fairly soon obtain it. :)

Oh, good to know. Will start my item search at the website then in the future and head to the found location :p