There are many brands of 410 pistol, Thompson Center Contender and Encore, Taurus Judge revolver and The Comanche just to name a few. Any or all should be available at your local gun store. If not or to do some research go to www.gunbroker.com and have a look around there, the site is loaded with about any firearm you could dream of.
Colt has never made a 45/410 pistol
$130.00 New in box.
In their catalog
Yes, it is a 410/45LC combo derringer, and runs about $80-$100.
chamber length
The Taurus Judge runs about $650 MSRP
Try Taurus
None. While there are firearms made to shoot .45 Colt AND .410 shotshells, they are designed and made to do that. A regular .410 shotgun is NOT made to do that safely.
I believe the Thompson Contender could be found in .410. There was a company from Argentina which manufactured a single shot pistol which, by replacing the choke, could fire either a .45 Long Colt round or .410 shotgun shell. As far as I know, they're no longer in business, and you'd have to search for one which was for sale.. offhand, I'd recommend Gun Broker, Auction Arms, and Guns America. You'd have to buy it there, and have it shipped to an FFL for receiving.
Yes, one gun manufacturer at least built a two barrel derringer that was chambered for 410 rounds. While a 410 is relatively small, technically is is still a shotgun, and in a pistol as small as the derringer, firing it is a wrist-breaker.
50-400 usd
There have been several handguns made that fire .410 shotgun shells- most recently the Taurus Judge. By the way, .410 is actually a caliber of shotgun, not a gauge. A .410 shotshell is about 67 gauge- but no one uses that designation.