You can still buy them from various online retailers, including Amazon. If you only want the foam tips or the newer, tapered gel tips, you can find these pretty much anywhere. It's harder to find the original (spherical) gel tips, but there are sellers that still have some stock of these too. Here's my experience with the various tips (from best to worst):
1) Spherical gel supertip - these are easily the best tips for any Monster Turbine!
2) Foam supertip (not as comfortable as the spherical gel tips and the sound quality takes a slight hit)
3) Sony hybrid tip (comfortable, but has a tendency to slightly over-emphasize the bass with these headphones)
4) Original silicon triple-flange tip (fair isolation, have a tendency to fall off the post and get lost in your ear canal!)
5) Single-flange tip (again, isolation is only average)
6) Comply foam tips. Although I love Comply foam tips with other IEMs, in the turbines they cause an unacceptable treble roll-off.
7) Newer, tapered gel tip. These are a disaster. Good luck getting them to fit or stay in your ear.