TA Performance sells these, http://taperformance.com/proddetail.asp?prod=TA_1723C My engine uses the High Performance Felpro gaskets, https://www.summitracing.com/parts/fel-1125
Try SK Speed. https://www.skspeed.com/felpro-1125-head-gasket-buick-400-435-455-4-385-bore-041-thick-each/
FWIW, I originally used the 1125's and after 4 years they started leaking coolant out the ends, teardown showed faulty gasket. JW says he has seen this issue before and advised I use the regular felpro blues.