Hi guys, I think I have a really stupid question here. I am building a small 496 Gen 5 block and I just acquired my crank.

Looking at it, I show it has the slot/indention for the key in the snout of the crank, but there is no actual key in the slot. Do I have to buy one of these separate?

yes you do, the machine shop that assembles your engine will have one :)

Thanks a bunch Vince. That alleviates some stress from my shoulders haha.

If I was one of the people that like to have everything before they hand off to the builder, how/where would I pick one up besides a machine shop?

IIRC, it's a 3/16" x 3/4" woodruff key. At least that's whats in a GM BB crank.
Any well stocked hardware store or even Home Depot will have it.

or another option if you wanna be extra safe (take it from my failed key experience that trashed the snout of my expensive forged crank) is a good auto machine shop will broach the keyway full length or even put in 2 full length keyways to keep the balancer from spinning on the crank snout. ask your builder his advice.