Re: 375 Cheytac dies
You can get them from either Lawton or CH4D. You will also need a press that will accept these larger dies. The RCBS Rockchucker will work - you just need to remove the die insert at the top and these dies should screw right in.
Len
NOT CH4D! Stay away from them! I used to stand behind them, now I wouldn't let them pay me to use their shit.
And for a single stage press, get the AmmoMaster and get the dies cut large to fit it. It also takes inserts so smaller dies can be used. These rounds can take a lot of torque, you'll want a stronger press. Or a similar setup. Trust me. If you want to speed things up and need it for the volume, the Dillon BFR is what I use. The trimmer ALONE is worth it. They don't list .375 but I know they're making those calibers, so you may just need to call 'em.
The small single stage press you already have for smaller shit is great for other odd jobs, etc. I still use it despite using the BFR and a 650 mostly. But I wouldn't try and stuff a Cheytac in it.
Stay the fuck away from CH4D, far away. I've got a thread on here in Bearpit this summer about them (rape/attempted rape, not final yet) over seating stems on .50BMG dies. Bottom line, zero CS, zero morals, customer always wrong (accused me of also JB welding the goddamn stem to the die body!) won't come through on warranty, charged my card without my consent. Surprise! Then I get all these excuses, etc., when they saw I wasn't having it, then they turned brutal. Accusing me of shit, calling me names, you name it. DO NOT GO TO CH4D. They may offer some alluring prices on large caliber parts, but they're not equipped to service people like us.
If M2 is still in business, they're the best and if he can custom a .50 I'm sure he can make one in .375. They ain't cheap. You could also call Dillon and see if they know who makes 'em since they make the BFR press.
DEFINITELY call Cheytac! If anyone knows what quality dies work best, it should be the mfg. Or A quality mfg. that makes these. That's your best bet. For a rifle like that, they're gonna want a fired case so they can make a custom die for your rifle. That's what you want anyway, not an off the shelf one. That's what I'm using for .50 but it's an M2. Hoping like hell M2 is still making dies when I need 'em for this rifle.
Not sure of other large caliber die makers. There's more for .50 out there. Some stuff is better than others. Good large size dies ain't cheap either. I expect $1000 for a good seating die and probably $600 for a seater. I've got a custom decapper for .50 that cost me more now than it should dealing with CH4D. Man, you sure do get what you pay for and cheap shit costs more in the long run. These calibers are just expensive, get good shit so you can take advantage of it.
Good luck!