Home improvement stores such as Home Depot and Lowe's and specialty retailers such as Reslat.com carry replacement slats for vertical blinds. Replacement slats or vanes are available for Bali, Levolor and PVC vertical blinds.
Reslat offers replacement vinyl, fabric and solar slats and channel panel inserts. The company sells slats by size and color rather than by brand. Individual slats cost from $2.49 to $9.99 at Reslat.com, as of September 2015.
Lowe's sells packages of 13 slats. Their classic white vinyl vanes are $20.97 per pack. The honey pine vanes are $44.97 per pack. Two packages are enough to fit an average pair of sliding patio doors.