where can i find headphones that are comparable with the apple iphone 3gs headset?

There are quite a few headphones that are comparible!  One is the Sennheiser PMX 80 Sport that sells for $59.95
specifics:
The PMX 80 Sport II headphones give you the optimum fit and comfort of an ergonomic neckband and vertical transducer system. Their sweat-proof materials are built to handle indoor and outdoor sports with ease.

Features

• Ergonomic neckband system for the best possible fit
• Powerful neodymium magnets for balanced, detailed sound reproduction
• Sennheiser Basswind System delivers powerful bass response
• Meticulous construction for serious durability
• Reflective neckband to ensure extra visibility and safety outdoors
• Sweat- and water-resistant materials
• Washable under running water
• Single-sided cable clip

There is also the JVC Full size that sells for $36.42
specifics:

• RING PORT structure
• Large 40mm neodymium driver unit
• Excellent sound isolation obtained through sound-insulating ear pads
• Gold plated iPhone compatible plug
• 3.9ft pure copper cord
• 1.57'' Driver Unit
• 6.6ft extension cord and 6.3mm plug adapter included
• 2-way foldable design
• Frequency Response 8-25,000Hz
• Sensitivity 101dB/1mW
• Maximum Input Capability 1000mW(IEC)
• Nominal Impedance 32ohms
• Neodymium Magnet type
• Weight 5.26oz

Also good is the Skull Candy Ink'd which sells for $19.99
specifics:

• Compatible with iPod®, iPhone™ and other MP3 players.
• Unique detailing.
• 11 mm speaker.
• Frequency range: 20 - 20K Hz.
• Impedance: 16 ohms.
• Cable length: 1.3 meters.
• Plug type: 3.5 mm gold plated.
• Included
• Ink'd earbuds.
• Extra rubber tips.
• 1.00 oz.