Is it the connector or the header on th power board? Have you checked the back of the header pins to see if any of the solder joints securing the pins look broken, cracked or dull? If any of them look suspicious, just reflow the solder by melting that joint then put a tiny bit of fresh solder into it. Probably wouldn't hurt to reflow them all on this nearly 30 year old power board anyways....

If that all looks okay and you're sure it's the actual IDC connector, you can buy IDC sockets from Pinball Life and I believe GPE, probably the other usual suspects like Marco as well. They also sell the IDC punchdown tool which allows you to push the wire evenly into the blade of the socket to make a good connection.

Most likely your problem is going to be on the header pins of that connector.

Pull the board and look closely at the solder joints on the back of the board. They are likely to be cracked and making a poor connection. You can probably get away just resoldering the pins. Williams boards typically have short header pins and it is hard to make a good solid solder joint.

If you want to replace them you need a 15pin molex 0.156" spaced header pins. Ed at great plains sells them. I like the header pins that come on a long strip and you can cut to size. Locking channel is not required.

The female connector is either an IDC Punched on connectors or individual crimped on pins that are in a housing. If the IDC connector is falling apart replace it with molex style crimp on connector. You would need .156 crimp pins and a crimp tool. Both available at GPE.