Does any one know where I can get a blank (45 or 50 cal) to use to rebarrel a Remington muzzle loader? Remington only sells a barreled action. I checked with a few barrel makers but I haven't found any.

There's only one place I go to for muzzleloader stuff: www.trackofthewolf.com

If they don't have it, I probably don't need it. Maybe they can help out with a barrel??

Green mountian makes great barrels. They might help

Track of the Wolf does not sell barrel black specifically for a Remington in-line muzzleloader. They do sell blanks and you may be able to get an oversized diameter barrel and have it tuned down and then threaded to something more like what you have. Green Mountain may be able to custom make you one or make one closer. You could also try contacting Ed Rayl for a custom barrel, the folks at muzzleloaders builser supply or Track should be able to get you his number. Longhammock also makes or used to make custom barrels (Longhammock 352-748-7373.. ask for Riley). Track may be able to point you to someone else that may help.