Forster makes 'em. Midsouth Shooters Supply wants $25.17US each. Currently on back order though.
Direct from Forster might be better. $36 each though. Also on back order.
Where you are matters. There's a $30 dollar US “base rate” for shipping/packaging if you order direct vs a local distributor if you're not Stateside
Headspace Gages - Forster Products