Aldi have O rings on offer 10th Jan. Out of the 420 in the kit there are probably a few that will fit the cornies.
Careful - I got these before thinking they'd come in handy.
If you read the box (on the ones i got anyway) it says dont use in drinking water or foodgrade applications.
I knew straight away though by looking at them they were that crappy black material - as apposed to the silicone - eg around a kettle element.
I have some o-rings for the top of the ball-lock on the corny keg if anyone needs any. I dont have any of the other o-rings but the ones on the ball-locks seem to be the most replaced.