The default location for PS bundled IRs is program files\cakewalk\vstplugins\perfect space. If you've somehow deleted them you can reinstall SONAR and get them back.
The IRs you get with PerfectSpace are quite serviceable, but it's fun to collect them from around the web. Voxengo's got a few free ones
here.
Noisevault is another good source.
I have hundreds of IRs in my collection, but haven't been out looking much over the last couple of years, having returned to algorithmic 'verbs for most applications. PS is a fine plugin, but a) it's not a true stereo effect and b) it's a CPU hog (as all convolution reverbs are).