First of all Cycle gear has 170/80-15 tubes for $12.99 or so. Never thought about the valve stem when going from a tubeless to a tube. I mean isn't the hole in an alloy rim much larger than one designed for a tube? Maybe not. If not then why would you need to buy a valve stem if you want to slip a tube under your tire?
Lastly, you want the tube for added insurance against leaks?
The tire should be plugged patched. Patched from the inside and plugged through the carcass. To do this the tire needs to be dismounted. Then you have to find a shop with the patch plug that will do it for you. or purchase a kit that alow YOU to do the same thing, without breaking down the tire.
Stop & Go's
Pocket Tire Plugger plus CO2 Inflation This kit includes four 16gram CO2 cartridges. It takes 5 cartridges to inflate the rear tire enough to hobble to a pump.
I recently ordered Stop & Go's
Pocket Tire Plugger - For All Tubeless Tires. Since this kit doesn't include inadequately sized CO2 cartridges, an order was placed for
genuine innovations monster air chuck which includes two 45gram CO2 whippets that will inflate the same tire in the place of five, 16gram ones. Kinda big $$ but should one on the road repair be needed, the cost savings alone more that recoup the what would now seem as a paltry up front cost.
Stop & Go uses mushroom heading plugs that seal from the inside out and through the carcass. imho much better than a simple gooey black piece of rope!