Hello,
I think you need to be a bit more clear about what you mean when you say "stability". There could be any number of meanings to this: stability in software, stability in control, stability in hardware and its supply.... etc.
As far as purchasing Lisa/L, there are currently no vendors making a finished board. I believe PPZUAV is currently selling a bare PCB for the autopilot, but you would have to assemble it yourself. If you are just getting started with Paparazzi, you may want to look at different hardware, like the NavGoV3 or the Lisa/M v2.0. Take a look at the wiki for more details on each.
Thanks,
-Stephen Dwyer