Hi,
Welcome to A.Org!
According to AmigaKit's web site, they have
A1200s in stock...
As for OS3.9 on an unexpanded A1200... You might be able to coax it into running, but it may be a bit on the slow side...
If you're not planning to expand to a graphics card-based Amiga later, etc, I'd probably stick with OS3.1...
- Ali