I just had to get these myself. The HELP! ones is a fix it kit for 1 stud, came with 2 different length studs, 1 spring and 1 nut. And it was $8. And the short one was the wrong length anyway.
My local parts store had them loose in a bin, so I got lucky, the dealer was next. He said he just buys them from the dealer and keeps them, because so many people ask. $12 for 3 studs, springs and nuts. They are all very different depening on emissions, exhaust setup. My right side on the '86 c10 had 1 short, 2 long because of a little emissions actuator mounted there.
They are a BITCH to get out of your manifold too especially when broken. I had to use a welder and a spare nut, placed the nut over the broked stud, welded the piss out of it, waited for it to cool, and finally got it to move.
Thestuds are are 3/8", but they have special threads on them to allow them to thread in, but not back out. The nut is a self locking type too.