Pub/bar cellaring sites stock cleaning sockets but the cornelius keg is only used in homebrewing as far as I'm aware so your best bet is to speak to cornelius directly: https://www.corneliusbeer.com/en/product/cleaning-socket
Alternatively try homebrewing sites such as MaltMiller or BrewUK.
Before you recreate a proper line cleaning solution as used in bars, ...
I don't know where you're located, but speaking as a professional brewer, I'm willing to bet if you asked your local brewery (especially one you frequent!) for a pint Mason jar of peracetic and offered to pay for it, we'd be more than happy to help out.
Bring your own pint jar :)
I have never seen this in less than 25L containers.
This is widely used in professional breweries, we buy it in 30kg containers from Murphy & Son.
NicheSolutions also carry it in 25L tubs - Percid 5 25L
If you are using PAA use it at a 3% ...
Sanitation is critical with each and every batch. Anyone can get lucky a few times. But eventually you will have problems if you don't properly sanitize your fermentation equipment.
Metabisulfite does not need to be rinsed or dried. If you are concerned about it going airborne or producing sulfurous gases, consider an alternative no-rinse sanitizer such ...
I typically return my sample after taking a reading. If your sanitation is appropriate then you wont have any issues. However, you mention that you rinse with water after sanitizing. I dont believe that's ideal, you are potentially reintroducing bacteria. Once soaked with the sanitising solution, you just leave it dry. That being said I use starsan rather ...