It's almost impossible to get the old stuff now, so you it doesn't realy matter. You'll need to use 134A in your AC system.
134a replaced R12 about 1995.
for a 99 it should be the new type AND there should be a sticker up on the inside of your hood stating the specific type in your system now.
R-134a
The 2012 Toyota Avalon has double overhead cam (DOHC).
The 2007 Toyota Avalon has double overhead cam (DOHC).
The 2005 Toyota Avalon has double overhead cam (DOHC).
The 2003 Toyota Avalon has double overhead cam (DOHC).
The 2013 Toyota Avalon has double overhead cam (DOHC).
The 2008 Toyota Avalon has double overhead cam (DOHC).
The 2001 Toyota Avalon has double overhead cam (DOHC).
The 2011 Toyota Avalon has double overhead cam (DOHC).
The 2004 Toyota Avalon has double overhead cam (DOHC).