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, let me just say as someone who also worked in the pub trade in the UK and now homebrews I found that the following homemade solution to cleaning lines is easy. You have to go line to line to clean so it doesn't work in a typical pub environment due to the number of kegslines to clean, but when I have a maximum of 2 lines to clean it's simpler and easier. I don't imagine you're going to have 14 lines to clean in one go or are walking up and down from cellar to bar.
The method I've used in homebrewing is as:
- Buy 2 small pressurised cleaning bottles. For example, https://a1barstuff.co.uk/5ltr-beer-line-cleaning-bottle.html
- Mark 1 as water only and use the other for line cleaning solution such as pipeline.
- Use the line cleaning bottle for line cleaning. Go from line to line and pull through as normal. Use the water bottle for rinsing out when clean.
These bottles are made for proper kegs rather than cornelius so your same dilemna will apply as before, so you will still need to talk to cornelius about getting the correct socket to fit to the bottle.