OWB:
I like the safariland als system. It comes with both paddle and belt attachment and is level 2.
http://www.amazon.com/Safariland-Ber...1&keywords=als beretta px4
IWB:
I have settled on 2 new and affordable holsters. The FOXX and the TAGUA IWB.
http://www.amazon.com/FoxX-Holsters-...s=foxx+px4+iwb
Looks like the tagua is out of stock on amazon. I actually picked mine up locally anyway. It is pretty good quality.
There are plenty of accessory options. It certainly isn't Glock where you can find accessories at the Quick Trip or seven - eleven, but there are after market sights, holsters, and even mags. And the mags made by MDS are fantastic.