You need to purchase some plain stabilizer -- cyanuric acid. That is still usually sold unblended or diluted, but we are beginning to see problems there, as well, so pay attention to the label when you buy.

To get a 60 ppm increase in stabilizer, you need 15 lbs.
Here are links from Amazon:
Kem-Tek Stabilizer 4lbs - $4/lb - Aug 2012
Nava Stabilizer 4lbs - $3.70/lb - Aug 2012
Nava Stabilizer 8lbs - $4/lb - Aug 2012
And, Walmart links (buy locally, but you can check stock online):
Aqua Chem Stabilizer, 3lbs - Caution: price not listed.
HTH Stabilizer and Conditioner
- listed at $16/lb!
Ironically, it's actually cheaper to buy dichlor, which is effectively 55% chlorine and 50% stabilizer (no, not a misprint -- a chemical oddity I'm not going to explain here) when you consider the value of both the chlorine and the stabilizer. And of course dichlor is much easier to use than the very slow to dissolve pure cyanuric acid.
24 lbs PoolBrand bagged shock @ Sams Club - ~$2.50lb - Aug 2012
Kem-Tek Dichlor 22 lbs @ Amazon - $3/lb - Aug 2012