lol. nowhere.
it's been a problem for a few years now. they used to sell them at k-var for like 15-20 bucks but no more.
for my recent conversion it was actually cheaper to buy bulgy ak74 kit and scavenge it for needed parts than buy sight block by itself, i sold unused parts on ebay and gunbroker and recovered most of my money. as much as i would love for such place to exist - it does not. sorry dude.
alternatively, if you already have M14 threads on the rifle, you can go much easier and a lot cheaper way by buying M14 to M24 adapter which is very cheap, like around 20-25 bucks.