They were hard to get for a while, and Sig actually held up civillian production to meet military and LE contracts they had. As they were offering the P229/9mm at the same time, it really didn't leave much of a hole in the line-up. They are still made today, and are available both new and used now.
Mags are easy to find due to the large number of police trade-ins. As pointed out, the P226 mags work as well, they just stick out the bottom a little. P229/9mm mags will work. I've shot many a P229 mag, and some of my factory hicaps are actually marked "P229". The P229 .40 is a differnt profile and won't even insert all the way into a P228 frame. Factory mags should be available soon in a couple weeks anyway, and outfits like Mecgar should be offering them again as well. Both of those work great. I've used Pro-mags on the range for practice, and they aren't bad, but they're a little softer than factory/Mecgar, so you can get burs or other deforming at the mag catch hole.
As noted, the NATO standard round is +P or higher compatible. The P228 is US Army standard issue as the M11, and fully capable of NATO standard/+P ammo.
I've carried the P228 off and on for quite a while. I still do if I think I need more of a gun than the six-shooter I normally pack. It's easy to live with, easy to use, easy to shoot well with, and works great.
They really are fine guns.
Ross