Hello,
I was looking at doing my own tank about two months ago. I ended up just picking up a used tank and stand off of craigslist to save money (i got a 29g with welded steel stand for $30). I found that the cost of doing my own tank and stand would have been pretty high and even more time consuming than just cleaning up a rusty metal stand and heavily crusted tank.
i digress... The prices i got for glass were
1/4" $3.75 per square foot
3/8" $7.75 per square foot
and $.10 per linear foot for edge finishing
my cost for glass alone to build a rimless 24" x 16" x 16" would have been $135. It did not make any sense for me build my own. I am sure prices will be lower in your area as i live in a relatively small town in northern Michigan, Look in the yellowpages of a phone book for "glass" to find suppliers. I am not entirely sure of your budget but you may want to consider starphire glass or low iron high clarity glass.
If you do decide to build your own glass aquarium, the silicone you should go with is
Momentive RTV108 or 103 (scroll to the bottom).
another glass thickness calculator is
This One. it is all in metric which is a little annoying but it allows you to control the "factor of safety". if you would like to build a rimless tank, which i am not sure is feasible on a 6' or 8' tank, then a factor of safety of 7.8 should be used.
You may want to consider acrylic, it is lighter than glass and is considered to have higher clarity. check out
This Thread.
Good luck in your endeavors.