Level IIA is probably enough in most places in the USA, here in Finland the choice is mostly level II, because 9mm is a common caliber and FMJ mostly used. IIA is rated for 124 gr 9mm FMJ only in subsonic velocities, and that means all common supersonic 9mm FMJ rounds (all loads above IPSC minor floor or IDPA floor) might penetrate it.
Contrary to what DerGlockenpooper says AP rounds are made in common calibers, like in 9mm, and can be shot from normal guns. However, AP ammo sales are limited to military only in many countries, and besides the USA they are restricted at least in all European Union countries.
The best penetrating common pistol caliber is probably 7.62 Tokarev, that can penetrate even level IIIA, and with steel core ammo IIIA also with extra trauma plates. European vest makers list it a serious threat for Russia, other ex-East Block countries and China, because all those countries produced Tokarevs (and Czechoslovakia the even mome penetrating CZ-52) in quantity.
BTW, in 1986 the Swedish prime minister Olof Palme was assassinated using S&W .357 Magnum revolver with US made armor piercing ammo (don't know the brand). Probably the shooter wanted to make sure he succeeded even if Palme had a vest, which he did not have. The murderer(s) is/are still at large.
Ossi