10I want to buy a cheap yet comfortable office chair for $100. Any recommendations where to buy it from?

Staples has some really great Office chairs, many at amazing prices!  One is the Montessa Luxura chair selling for $59.99!
specifics:

• Luxura™ faux leather seat, back and padded arms, has the look and feel of leather, available in black or brown
• Fixed padded arms
• Built-in lumbar support
• Pneumatic seat height adjustment
• Tilt tension with tilt lock
• Chair Dimensions: 36-1/4" - 39-3/4"H x 26-1/4"W x 28"D
  Seat size: 19-1/4" - 22-3/4"H x 19-1/2"W x 17-3/4"D
  Seat back: 18-3/4"H x 19-1/4"W
• 2" hooded casters with resin base
• Weight rating: 225lbs.
  Usage: Up to 3 hours of continual use per shift
• 1-year mfr. limited warranty
• Staples Brand 100% Satisfaction Guaranteed

There is also the Belmore Leather chair that sells for $69.99!
specifics:

• Luxurious Black leather seating surface
• Loop arms
• Built-in lumbar support
• Pneumatic seat-height adjustment raises and lowers chair quickly and easily
• Seat and back angle adjust simultaneously and lock for comfortable positioning
• Swivel controls allow user to move freely
• Tilt lock keeps chair in comfortable position
• 39-3/8" - 43-5/8"H x 25-1/2"W x 29-1/2"D
• Seat Height: 18" - 21-1/4"H
  Seat Size: 20"W x 19"D
• Ready to assemble
• 15-year mfr. limited warranty

There is also the True Seating Concepts chair selling for $99.99!
specifics:

• Black Puresoft™ faux leather with quilting detail
• Padded arms
• Built-in lumbar support
• Waterfall seat promotes better leg circulation
• Pneumatic seat-height adjustment
• Tilt allows seat and back to recline
• 40-1/2" - 44"H x 25-1/2"W x 28-3/4"D
• Seat Height: 19" x 22-1/2"H
  Seat Size: 19"W x 17 1/4"D
• Meets or exceeds furniture industry (ANSI/BIFMA) standards
• Ready to assemble
• 2-year mfr. limited warranty