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Buffalo Arms seems to have both .405 and 40-70 SS in a variety of brands.
Although I see Hornady's are out of stock.

Think I found the Hornady .405 on Cheaper Than Dirt last month when I was thinking of the .405 rather than the 40-65 for my new CPA barrel. Check all the 'usual suspects' (Graf & Sons, Natchez, Mid-South, etc.).

MikeT,
I am just getting started with this sport and am relying on recomendations from others with more experience.  My Hepburn is an original and, according to Dave Crossno, the head space on originals is different from reproduced rifles.  He told me that, while the .405 fits new rifles, the head diameter and thickness will need to be reduced to fit an original chamber.  He said to load a .30-40 case with 6-7 grains of smokelsee, fill with Malt O Meal, seal, and fire.  The result will be a casing properly formed to fit the chamber.  This is either a clever way to resize a cartridge or it is a standard joke on a newbe.   

The rifle arrived today, looks wonderful, and I want to shoot it as soon as reasonable possible.  I have Cerrosafe coming to cast the chamber and barrel so that I will know what have to work with.  But I would like to get my hands on a few .30-40 Krag to test how their fit.

are you sure your 40/70 is based on 40 cal. brass? every remington I remember in 40 cal was based on 45 cal. brass except one short range RB that was closer to 40/63 Ballard than anything else. if you are new to this game and haven't had a chamber cast made you really need to consider doing that first. don't buy bullets, molds, brass or dies til you do. if you have already confirmed exactly which chamber is in you rifle then I'm just shooting my mouth off here but 405 based brass in old remington seems unusual to me.
kootne

I know what it's like when you get a new rifle but Mike T and Kootne have given you some good advice. Wait on your purchases until you know more about the chamber.  I have seen several Hepburns in 40-70 SS and did own an original in 40-63 Ballard.  Get a hold of some .40 cal. round balls and flatten them a little with a hammer.  Oil up the bore good and drive a ball through with wooden dowel rod.  I expect that you will see that it has an odd # of grooves in the barrel.  They are hard to mic but I have tightly wrapped a thin .002" feeler gauge around it, miced it and then subtracted the difference.  Now find the twist by putting a tight fitting patch on a mop and pushing a ball bearing handled cleaning rod through and find the twist.  You can wrap tape around it and see how far the rod goes in while making one revolution.  It will probably be around 18-20".
Now take the end of a dial caliper and extend it a little to measure your rim thickness.  Hornady brass will average around .070".  You may have a lot of work ahead of you if you are going to turn them down.  I think you will find that the stretched Krag brass from BACO will work best.  
When you get your chamber cast finished, hopefully, the neck of the chamber will measure at least .430".
Let us know what you find out and we'll go from there.  Bob

It looks like Cabela's has Hornady .405 brass in stock.