Any places where I can buy a big bore breech?

That's because a bolt for a .375 is going to need better threads, more leverage to cock than a factory Mrod.

If you know anything about machining, or have a machinist friend with equipment, you're way ahead of the game. Do you know CAD? You could design your own. Maybe someone like John Bowman, Extreme Big bores, Gargoyle rifles, Mr. Hollow Point would be able to get you what you're after. An older Rapid solid block reciever would work well. 


 
Jefferson State Air Rifles - JSAR, has done some custom machining like that but it's not a listed item on their website, you have to call and talk to them. Good guys.



I've got Jim Gaska at Wicked Air Rifles building a custom .357 using a bored out action to 5/8 barrel OD and a different retainer system... sort of a prototype. Trying to get him to produce a 'Magnum' action with longer bullet loading port that could use Bulldog magazines but he's so busy producing WARP's and FLEX's probably not happening.