Hey Will, I'm gonna tell you something you probably don't want to hear, but you need to.
If you want to play with these old hammers, and other machines, you're either gonna need to develop machining and fabrication skills, or make good friends with someone who already has them, and the equipment to make parts.
I say this, from experience, not sure what you paid for the hammer, but you could easily find a shop that'll charge you half, or as much, just to make you the die. Even the 75 lb Bull I ran for years, which is only about 20 years old now, I had to fabricate new guide carriers after repairing the originals multiple times, replace numerous switches, hoses, and minor assembly pieces, more times than I can count.
I've already had to make new keys for the dovetails of my Bradley strap hammer, and I only recently got her running after putting her in place over a year ago, still need to pour babbit for the eccentric, and build a few minor replacement parts that I know are wearing out, and these are probably the most bulletproof/industrial grade power hammers ever built.
I'm not saying you won't occasionally find somebody that has a broken down Fairbanks (they are out there) that they're willing to part with a piece of, or maybe someone that has a few parts, but be aware that you are going to have to regularly repair/replace components over the years if you run them hard, and I want to go ahead and get your head wrapped around this idea.
Dovetailed dies are gonna be a PITA, and most machine shops aren't gonna want to mess with one offs, or will want an arm-and-a-leg if they are willing. Most of the hammers use different sizes and configs, there's typically no "off-the-shelf" options at all. Your best bet however, is checking some of the blacksmithing forums like IForgeIron, where there are a lot more old mechanical hammer enthusiasts, or the FB groups. Find someone that has a few of these hammers and you'll likely find someone with a manual scanned, and/or schematics of various parts, and likely the specs on the dovetails for dies.
If you do have to have some machined, I'd recommend getting some die "blanks" made, which consist of your dovetail, and a flat top plate, that you can weld various die "blocks" of whatever geometry onto. Getting multiples of a one-off part made, are usually not much more expensive, or at all, than having one made, since most of what you're charged for is processs/setup/design work, not the actual machining and materials initially.
If you don't know anybody with the machinery and tools to make them, check some of the forums that have manual machining enthusiasts, like some of the sub forums on PracticalMachinist, where you've got hobby/job-shop guys running old manual shapers, and big old horizontal and manual mills, as they can often produce odd-ball dovetails cheaper and more willingly than any modern CNC outfit will.
You may be able to find existing dies on IFI or similar though.
What is wrong with the bottom die? Are you sure it can't be repaired or built back up? If you've got a complete dovetail, or even two pieces, it likely can.
