First off, does anyone know where to find factory Browning 9mm barrels?

Secondly, can somebody recommend a good aftermarket barrel. Preferably drop in. But also, how hard is it to fit a barrel? Do you alter the frame, or the barrel?

Brownell's listed the factory Hi Power barrels for $292.71 for 9mm and $301.81 for 40 cal, pre gunsmith discount.  
www.brownells.com

Have you tried Bar-Sto for a replacement barrel?
www.barsto.com/Beretta_Browning.html#anchor496679

Thanks for the info. Actually Bar-Sto were the replacement barrels I was looking at. Opinions of these anybody?

they are supposed to be very nice, but I have yet to wear the barrel on my HP out yet.

I haven't used a Bar-sto in a HP, but I've seen a couple in action.  They did require slight fitting, but nothing hard.  Easy enough to be called "almost drop-in".  

Preformance was greatly improved over the stock military barrels.  They shot fantastic.  If I were to wear out one of my HP barrels, or if I was going to build-up a HP package, I'd use a Bar-sto.  They've been making them for decades, and they've been good for that long.

Ross

Oh as for fitting, usually you do everything to the barrel itself.  You generally always modify the easiest to replace/cheapest/replaceable part.  The barrel is that, and the frame certainly isn't.  

Usually there will just be a little extra metal in various places like the hood, lug, etc.  When you try to drop it in, the slide won't lock.  You just adjust where the barrel needs it until it works the way you want it.

Generally speaking, the looser the more reliable, the tighter the more accurate, but there's a point where too loose isn't good.  If you take your time and are careful, you should wind up with an accurate AND reliable handgun.

Ross

Bar-Sto barrels are of a much higher quality than the factory barrels.  The tighter fit will get you a higher level of accuracy as well as the tighter manufacturing tolerance.  Also, they are VERY rust resistant.  Have a gunsmith fit it if you dont have any experience fitting them.  Its a fairly involved process and if you screw it up, you dont want to be out of a barrel.