Where to purchase

It would help if you let us know what you plan to put it on. Do you just want a blank that can be machined to fit or do you want a barrel for a specific rifle? There are several vendors who sell blanks and others that cater to a specific brand of rifle. More info will help you get a better targeted answer.
Just about any one will work as long as it's cheap. I do all my own machine work. Most I looked at from a retail store were anything but cheap
 
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Just an outside idea...

If it’s just for testing power and basic function, not accuracy, you may want to look at DOM tubing and see if there is a size that will work. Or a piece of brass tubing set into steel tubing with threadlocker or bearing compound.
That's a good idea. That will be the last resort
 
Call crosman and just order their crosman barrel. Last time I order one for the marauder it's only $20.
I didn't think they made a 30 cal marauder. Couldn't find one on their web site. I know that some barrels are out of stock and they are not replacing them. Tried getting a 14" in .177, they sent me a .22 When I called I was told that the .22 was the updated replacement. It was of no use to me and wanted to return it. They told me to just keep it and refunded my money.
 
I didn't think they made a 30 cal marauder. Couldn't find one on their web site. I know that some barrels are out of stock and they are not replacing them. Tried getting a 14" in .177, they sent me a .22 When I called I was told that the .22 was the updated replacement. It was of no use to me and wanted to return it. They told me to just keep it and refunded my money.
They might make one for one of the Benjamin big bores. I wouldn’t be surprised if they make a bulldog in .30.