So what could I expect to pay for a one? A used one? Where could I find one? Where could I find load data and reloading supplies?

I can only generalize on prices, but, based on my limited experience, 7k to 15k. The bigger the name, the higher the price, more engraving= more money. Same for overall wood quality. I bought my 450/400 3" from Champlins for $7500. It is a Rodda, good solid working gun, border engraved, fairly plain wood and is an extractor gun. Roddas were well made Birmingham guns, but do not have the cachet of names like H&H, Jefferys,etc. Army & Navy is another that generally does not command as high a price even though they were high quality guns, they and Rodda and Manton were retailers that had their guns built by others with their name on them. I have been watching the prices carefully for the last 2 years and would say that you need to plan on spending 12k and hope you get lucky like I did, and find one you like for less. If you do not get in a hurry, it will raise your odds of sucess. I would also call George at Champlins and tell him what you are looking for and the price range that you have, maybe when something comes in he would give you a call. You can got to Champlins site, Cabelas, Gunsamerica, there are many to choose from. You can find load data here and on Nitro express.com. A-square has some in their manual, but this is a very popular cartridge on this site and we will be happy to help you out if you get one. I bought my dies direct from C&H and I am pleased with their performance and they were $100 less than RCBS and shipped that day, no special order. Brass and bullets are generally available from Huntingtons, Midway, and Buffalo Arms. I am sure others will have more info. You could also consider having Butch Searcy build you a new one for around the same money as a used English. They are good guns, I just had to have English! Good Luck and keep us posted! Lee.

And Jamison is in the middle of a run of brass for both versions of this caliber right now. It should be ready for sale with in a week.

Most of the used ones will, of course, be British guns and, unless your interest is collecting, you'll be looking at boxlocks, so I'll confine my comments to those.

Condition is paramount, especially the bores. Fifteen years ago, .450/.400s in great shape were a dime a dozen, and NOBODY wanted them. Today, they're extremely popular again, and it hasn't taken long for the market to soak up the nice ones. Most of what you'll see on the market today will have problems of some sort. Nice Brit .400s are out there, but you have to be selective. Contact the seller as soon as you see one come on the market. The ones that have problems or are overpriced hang around, the ones that you're looking for are gone fast. A good friend finally got a nice one last week. He looked for 10 months.

Forget the name. Condition is much more important, and the identity of the trade maker who actually built it is more important than the name of the retailer that ended up engraved on the gun. Virtually none of the big name London firms made their own boxlock DRs. They had them built for them, mostly in Birmingham. Most boxlock DRs from Purdey, Evans, Rigby, Army & Navy, & Gibbs came from Webley & Scott, and the Webley Screw-Grips are arguably the finest boxlock DRs ever made. Every Holland & Holland boxlock DR I've ever seen was a Webley, but so were those from London Sporting Park (ever heard of them?). Same gun. A friend of mine who posts here has a .400 from LSP and it's a superb rifle. Most Jeffery DRs came from Harry Leonard, another top quality trade maker. John Wilkes also made quite a few good quality DRs for the trade. Cruise through the old strings at NitroExpress.com and try to learn to identifty the various trade makers. If you run across one you can't figure out, ask someone who knows. When you find one you think you like, ship it to a British or European trained gunmaker (never a gunsmith) for evaluation. Be prepared to send several back before you find the right one.

Price wise, for a good quality .400 in nice shape, expect to pay $12,500 to $17,500. Exceptional rifles can bring more. A friend recently bought a really nice .400 from one of the large dealers for $10K. The dealer didn't recognize the name (it was actually a good one) and badly underpriced it. So yeah, you can get lucky, but don't count on it.
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"Serious rifles have two barrels, everything else just burns gunpowder."