Areas to find rust are:
*Torsion boxes, qiute often missing all together.
*Rockers, both inner and outer.
*Kick panels, pretty much always find some there.
*Floors, depends on the life they were treated to.
*Rear bed sills, yep look there also.
*Wheel wells, same story.
**Windshield frame** < nuff said, make sure to check 'em.
*Firewall behind brake booster.
*Cowl, can't really see it until you remove the fenders.
*Lower door pillars both front and rear.
Blazers/Jimmy's have an uncanny nack of hiding the rust, so if you do find rust and you will, it's typically only the beginning.
Take your time and find a nice one, it'll be well worth it in the end.
They seem to be all over out west, be prepared to make a road trip for the
right one.
Good luck and welcome to the board, enjoy the ride.