In older trailers (we call them vintage) you will find the costs for the shorter ones are higher than the longer ones and they are harder to find. There was a very rare bunk option (actually a canvas sling in an aluminum frame) made for Airstreams with twin beds, but they tend to be at least 25' long, so not much different than what you have now.
If you are looking at newer ones, there are quite a lot of 16, 19 and 20' Airstreams which have been made in the past 8 to 10 years. However, I know of none of them with bunk beds and can't recall anyone who has added bunks, but I sure could be wrong on that.
How to find one that is in good condition is always going to be a problem, be it a dealer or a private sale. You wind up putting trust in others to evaluate condition, and what you think is good vs. what someone else thinks is a common issue which is hard to resolve. Add to it the problem of scams and problem international sales and you are not going to have an easy time finding what you want, I fear.
I do wish you best of luck in your quest. How about a vacation to the US to find an Airstream? Might be a fun, but expensive way to do it.