Looking for test leads for individual PCB header pins or socket pins
For experimental purposes, bare crimp type contacts for comparable diameter posts can be used individually, covered in heat shrink tubing rather than inserted into their usual housings, provided that they are the style where the contact surrounds the post sufficiently to grab it, rather than needing the plastic housing to hold the post against a single sided contact.

There are now import/discount suppliers selling ribbon cable terminated in plastic molded single male or female .100" spacing .025" post contacts, with the idea that you can manually separate the ribbon as desired, however quality/reliability seems variable, so you'll want to test any setup with those for integrity before assuming your problem is more complicated than a failed connection.

In a situation where failure or accidental shorts would not have lasting consequences, I've also been able to crudely break out an XLR using small alligator clip leads in order to take some quick measurements.