In addition to thru-hiker
I found an Ebay store Shapiro Supply that's a good Ti source for a wide variety shapes and sizes that aren't readily available elsewhere. Of particular interest may be their large diameter, thin wall tubing (e.g. for use in an ultralight wood stove). They have various grades of the stuff, so you might want to check on McMaster-Carr for the properties of each.
Are there any other good sources out there that include materials not covered by these three, or that offer them for less? I've also heard of an elusive, once made Ti MSR bottle. Does such a thing as a Ti bottle exist?
Also, I've read the theoretical "how to" on welding Ti, but does anyone have actual experience with the stuff? I'm thinking about getting a welder(for this and other projects), but it sounds like this stuff is a real beast to work with.