I know this post is fairly old, just came across it researching which pH meter I want, but I might be able to help. The jack on the right is called a BNC, so I'm using that as a size reference. It's possible that is an RCA jack, I can't say for sure without seeing what the probe looks like.
I think what you're looking for is a 3.5mm TS jack locking nut. The TS stands for tip-sleeve, they're sometimes called a 3.5mm mono jack. You need an electronics parts store, not a hardware store. A music shop that repairs amps and guitars would have the right one. Your bio says you're in the USA, and my searching for a link shows that Americans might call this a 1/8".
If you have nothing local, you could get a kit like this and just use the part:
Amazon.com
Good luck.. hope this is helpful if you didn't solve your issue.