What I do to *prevent* constipation is to keep a couple pots of grass in the
house. Actually, instead of pots, I use those plastic shoe boxes that they
sell in places like Walmart. I keep 2-3 and rotate them so there's always
fresh greens around. This is something cats eat naturally in the wild. The
fiber helps the hair and other stuff move through their digestive tract.
The only time Pepper gets hairballs is when she's not eating her grass -
usually because I let it die out without a new batch.
The stuff they sell in the pet supply stores tends to be expensive - $3 to
$5 for a tiny container. I buy oat seed (and sometimes rye seed too) in
bulk from Pine Tree Seeds - http://www.superseeds.com/covercrops.htm.
You can get 1-1/2 pounds for $2.95 or 5 pounds for $6.95. (Plus a flat rate
of $1.95 shipping). The smaller bag literally lasts for years.