I haven't found that to be the case. A 6.3 oz t-shirt is the same weight and quality as it was 30 years ago.
For a good deal and huge selection, try
http://www.cheapestees.com. Most of the shirt companies - Gildan, Jerzees, Hanes, Anvil, Champion, etc. - offers t's in different weights. Buy their heaviest, if that's what you're after. Try one or two from each company and choose the one you like best. There are small differences in fit and material for each one. Some are softer, some harder finish, different cuts in the neck and arms, etc.
It's a company that I imagine sells a lot to screen printers, but they'll sell individual shirts. I usually put in an order for eight or more at a time and get free shipping. They recently sent me a promo email with a 20% off code: B20X, but it ends the 13th so may have just expired.