Glass???

Go to any commercial glass supply company, such as ones that sell glass for residences, businesses, etc. You should get tempered safety glass that is at least 1/4" thick, and if your gear is heavy, then I'd suggest 3/8" thick (which may have to be special ordered). Have the company cut pieces to the sizes you require, and have them bevel the edges to eliminate sharp edges.

One last "tweak" you might want to consider: if the glass is going to be hidden underneath a component, but may be subject to picking up and passing on vibrations, then get a sheet of restrained-layer damping material, such as used in cars for car audio, and apply it to the glass. I used a material called DexDamp, which I got from a marine supply company, but car audio installers also a similar material.

Last, if the glass will be supporting a vibration-sensitive component, then put isolation devices such as Vibrapods or Sorbothane pods under the glass, either at each corner, or in a triangular pattern.

I have had excellent results with 1/2" glass (I use it on a Target rack for extra isolation) with vibrapods between the glass and the normal Target shelf as well as between the glass and component as Sdcampbell says. In this configuration (a vibrapod sandwich), much better than granite, marble or more mdf, all of which I've expermimented with. If you are supporting a bit of weight, I'd suggest the 1/2"-it is very dead-little or no "ringing" or tilted up high frequencies (at least to my ear) which you will get with granite or marble, and none of the muddiness that you get with MDF. If you have lighter components and want to save some money, you can actually buy security glass or "laminated" glass, which is much thinner, but consists of two layers of glass sandwiching a glue like material to prevent shattering. This in turn, really serves to damp out vibrations and eliminate ringing that is inherent in thinner sheets of glass. Laminated glass is also going to be a fair bit cheaper than 1/2". I use the laminated as well, also with good results. Again, as Sdcampbell said, use the vibrapods in conjunction with this.

Not so sure if this is such a good idea. I've seen several glass-shelved racks & always wondered how they can sound any good? I removed the glass doors from my own rack & the sound improved significantly. But if you must... at least use considerable isolation such as is suggested above.