The first generation of the GMC Suburban stayed in production from 1973 until 1991 with very few changes. The early models had engines that would be considered weak by today's standards, including a 6.2L 130 horsepower V8. Some engines were considerably more powerful, making upwards of 250 horsepower. The leaner engines were most popular on the Suburbans exported to Europe. I can only assume they were used on farms over there, because I've seen the city streets and there isn't any room for a vehicle this large in most places.
1992 saw a change in platform to the GMT400 for the Suburban. This was a major change, and many years late for General Motors. The 1991 Suburban model was essentially a 25-year-old vehicle made with new parts. The 1992 model showed that General Motors did want to move forward with their vehicles, something they've traditionally resisted.
Some of the improvements on the second generation GMC Suburbans include indepedent front suspension with coil springs on the 2WD and torsion bars on the 4WD. The Vortec engine introduced in 1996 increased horsepower while also increasing fuel efficiency.
There are plenty of Suburbans on the used car market, although fewer if you are looking only for the GMC variant. You can probably find one or two in your local classified ads if you live in a highly populated area. If not, you can always try eBay, where they tend to sell for even less than classified prices.