Jeg's used to list them (under SBC), but their latest catalog doesn't list them.
Anyways, a vertical locking link type is what you want.
Or just call Jack Cotton and get them.
I've used some from various sources, and there's been a thread or two about it, whatever brand you get, be sure to take them apart, and clean them thourghly. Some of the *shipping* grease is so thick that it won't let the plunger move...