You can try Dulcolax Tablets for FREE with the mail in rebate they have available for up to $10 back on a prepaid Visa card. It's $5.99 or $8.99 at Walgreens (depending on which kind you buy) and there are coupons available that would lower your up-front cost.
Dulcolax Tablets 25+ct, Pink Tablets 25+ct, Dulcolax Stool Softener 25+ct, DulcoEase Pink 25+ct. $8.99
Use $1.00/1 Dulcolax or DulcoEase 25+ct or Dulcogas 18+ctSS 7-26 (exp. 9-30)
Subtotal: pay $7.99 at Walgreens
Submit receipt for mail in rebate here
Fina price: FREE!
(Thanks Wild For Wags!)