Replacement Long Lee Enfield barrel
Joe, how much is the rifle worth to you? There is the cheap way, then there is the easy way.
The chances of finding a replacement barrel are pretty slim to none. Most spares have been used up, but you might get lucky, I'm sure that there is one here or there gathering dust. Most are found in Australia or New Zealand, which is where the Brits sent their obscolete rifles when replaced by the SMLE. The odd barrel shows up on FleaBay but usualy goes for silly money.
Best practice I find, is to buy a sportered rifle with uncut barrel and use it as a donor for parts. The parts left over can be sold, often for as much as I paid for the the donor rifle!

. A gunsmith might charge $100 to swap out barrels and do the headspace.
An alternative might be to get the barrel relined. I had one gunsmith quote me a ballpark figure of $175, and that would be if I dismantled the rifle and brought him just the barrel. I have been told that that is a good price, a liner would have to be custom machined due to the 30 inch length.
Removing the barrel and then replacing it back onto the receiver is not something that can be done in the home shop unless you invest in the proper tools. A barrel vise and an action wrench with correct spacers and bushings for the Long Lee knoxform and the receiver profile will run you about $300. Putting the barrel back on, it has to be carefully indexed to the receiver, otherwise sighting might be off or there might be extractor issues. It is not rocket science, but it has to be done right.
Easy way is to let a competant gunsmith could do the whole job, but it is the labour that runs up the bill. If that were the case, it might cost more to reline the barrel than the gun was worth. Cost might not be an issue if this was a family heirloom or similar. Relining would keep the rifle original barrel and its markings, which might be important on a rare piece.
And a third option. Is this an original spec rifle, or is it a sporter? An SMLE barrel is easy to find and would screw right on there.