Where do you purchase them?

and how hard are they to install

Globalintersection.com
Amazon.com
DriverFX.com
Kragen Auto
Pep Boys
AutoZone

These are just a few places to get the tips.

For exhaust tips, there are only 2 or 3 reputable brands that you would probably want to spend your money on.

Try to make sure you get Stainless Steel (T-304) quality tips.

I have Vibrant tips on mine and they cost me $64 total.

I had them installed locally for $40. ($20 each)
The shop that I went to charged me $30 for the custom collars that were needed to make it a perfect fit.

If memory serves me right, the stock exhaust was 2.25" and the back end of my Vibrant tips were 2.75" and they open up to 4".

The model number is 1303 for the 4" version and 1304 for the 3.5" version.

I wanted a rolled edge, double wall, angled, round, T-304, 4" tip and Vibrant had exactly what I wanted for a reasonable price.

Magnaflow also had a perfect product, but they wanted more than twice as much for theirs.

Good luck.

<center><img src="http://i146.photobucket.com/albums/r247/A4SHO/DSC_2217.jpg"></center><p>They are 4"x7.5" Double walled straight cut. I know the GM at the local Midas, so he did it for free, and I gave the guy doing it a $20. Took 20 mins. I like mine, but wish they were tucked in about half an inch when cold. Might get it fixed next summer.

<img src="http://pegasus.cc.ucf.edu/~mi167268/Car/magnaflow.jpg">

if you're gonna get hit by something it might as well be that.

Oh I was on foot and he blew past me doing a minimum of 120 on a very short small road :)