Where can I find a 2.5mm to 3.5mm adapter that will let my cellphone headset (with mic) work on an iPod Touch?

You can find that adapter at a couple areas!  One is at Everything iCafe where it sells for $9.95!
specifics:
This lightweight and convenient Palm (3.5mm to 2.5mm) adapter allows you to easily connect your standard 2.5mm mono headset or TTY device with your 3.5mm device.  The Palm Audio Adapter also supports the send/end button with most headsets. Adapter Specifications:

• MALE END = 3.5mm
• FEMALE END = 2.5mm
• LENGTH = 3 inches
• Supports TTY Devices
• Works with MONO Headsets
• 4 Conductor Plug

Applec Touch also has it for $9.95!

There is also an adapter/splitter which lets you and a friend listen!  This sells for $4.95
specifics:
The SPE Headphone Splitter lets you share your audio! Perfect for car trips, plane flights or anywhere you want to share audio with a friend! Simply connect it to your iPhone 3GS, iPhone 3G, iPhone, iPod Touch 2G, or iPod touch, iPod, MP3, or DVD player then plug in two sets of headphones into the splitter and you're ready to go!

• Installs quick and easy
• Allows multi-user listening
• Connects two headphones