Hello
This is what I have done on many doubles.
I buy a used set of Briley tubes.
http://www.gunbroker.com/Auction/ViewItem.asp?Item=131239992Some are cheaper and some more expensive, I consider these expensive.
Many times in the past I have spent less than $150 for a used tube set.
Then I measure the choke of the gun the tubes are intended for and using a lathe, fit the tubes to the barrels. This is not hard to do, if you are patient.
Latest project was fitting a set of 28 gauge tubes to a Baker damascus that has a fracture in the right barrel just ahead of the forend and on the 4 o'clock part of the barrel. The gun now is usable and shoots great at skeet with the screw in chokes of the tube set.
Let us know what you decide to do.
Mike