They still come up for sale every now and then. Not everyday common, but not super rare either.
How good are they?
In my opinion the M39 is the finest pattern variant of the entire Nagant line, excepting possibly the Czech VZ 54 Sniper model. Mine is a 1944 SAKO and has one of the finest quality barrels I've seen on a military service rifle. The triggers are very good also, being redesigned slightly from the original pattern, and have a proper 2 stage pull. The sights are very good, and the front sight is screw adjustable for windage. The M39's stock design is much more comfortable to me than the 91/30 pattern, and the overal length of the rifle seems to balance better as well. However, due to it's heavier barrel and stock it weighs a bit more than a 91/30.
If you can find one at a reasonable price in very good condition or better then buy it. You won't be dissapointed.