what is the best stuff to use with a carpet cleaner?

You definatly want to use a detergent that is specifically made for carpet cleaners!  Best Buy has the Hoover Anti Allergen Carpet Cleaning solution which sells for $9.99
specifics:

• Compatible with most carpet cleaning machines
  Including Hoover, Dirt Devil, BISSELL and carpet rental machines. Safe for colorfast carpets.
• Anti-allergen formula
  Features an allergen remover for deep cleaning.
• Scent and dye free
  To leave your home clean without irritating fragrances.
• On-board stain guide
  To simplify the stain removal process.
• 48 oz. bottle
  Ensures you have enough detergent to get the job done.

Kohls has the Rug Doctor Oxy Steam Carpet Cleaner selling for $22.99
specifics:
A breath of fresh air for your carpets. This high-powered deep-cleaning solution relies on heat-activated oxygen boosters to grab dirt and grime and lift them out of your carpets. And it covers a lot of ground — one bottle of concentrated cleaner makes up to 48 gallons, or four to eight rooms. Use it in nearly any deep-cleaning machines. Plus, it won't leave your carpet with any dirt-attracting residues. Due to its contents, this product cannot be shipped via our Priority Service or sent to Alaska, Hawaii, and/or APO/FPO military addresses.



Another is the Bissell 2x Ultra which sells for $21.99
specifics:

• Compatible with most full-size deep cleaning machines
  Including BISSELL, Hoover, Dirt Devil and rental machines.
• Advanced formula
  With Scotchguard for deep-cleaning performance.
• Helps protect carpets
  By revitalizing soil and stain protection.
• 60 oz. size
  To clean a large area of carpet.