Last season I had a valve 'go' on my Avon Redstart.
After some enquiries, they wanted about £25 for a valve, £10 for special two part glue, then about £8 for another type of glue. On top of that you had to cut the old one out.
I decided to go valveless and see how I got on.
When you pump up it is an interesting challenge in making the change from pump to plug without losing too much air. After a season, believe I now hold the world championship at this and intend staying this way for good. Deflation of course is a doddle. For best results do this at about midnight on the way back from the pub when the air is cool. When the sun comes out in the morning the thing will be like iron.
I have no idea where you get your valves serviced but mention this, as like most of us, you might start going down dead ends .
Inflatable boat center, somewhere in essex. Dont exactly know where, but my brother purchased new valves from them along with all the gubbins to fit for about £14 each. in saying that if yours are the 2 part valve where the valve bit comes out of a collar, there isnt much servicing to be done. Of course i may be wrong
There is a company called messenger marine based at Fishbourne near Chichester who do all kinds of inflatable repairs. I had new rowlocks fitted last summer and they did a good job for a reasonable price. They have a website too.
The bungs seem to be of the same material as the valve and came with the boat. Without the bungs the boat loses air quite quickly on just valves ( on mine anyway).
Sure you will be able to make something up if you need to.