i think there's parts for the Caddy 500 / 472. for the other Caddy stuff, yeah, not so much.
having the same spacing as the Rover v8, it is possible to use Rover intakes with spacer plates but most people think the Rover runners are too narrow to feed the 300ci.
so many People need them.
there are SOME.
but even among Buick fans, the 300ci is a low production engine with little availability. it was only in production from 1964-1967 and 1966-67 production was split with the 340ci ... which has an even taller deck height. so you can't use those intakes either.
some people have started to notice that you can take a Buick 350ci or 340ci crank and swap it into a 300ci block, which gives you a 340ci displacement engine with the 300s lower deck height and CG. there's a Hot Rod magazine article which covers this.
Rover v8s ( ie - metric 215ci variants ) were in production until 2004, so there has been a lot of interest maintained in the English manufacturing and hot rodding community from that end.
the major aftermarket American supplier for Buick is www.TAPerformance.com. they don't have any aftermarket 300 intakes, but they might have some old 4 barrels sitting around.
D&D ( that AluminumV8 i linked last post ) does a lot of Buick 215 / Rover building and they are even more likely to have a stock of Buick 4 barrel intakes.
i believe both of those companies will ship over seas.
The way to crush the bourgeoisie is to grind them between the millstones of taxation and inflation.
Government schooling is about "the perfect organization of the hive."
H.H. Goddard, Human Efficiency (1920)