Velauno (2430 Calle Loíza, San Juan, 787/982-0543 or 866/PR-VELA-1— 866/778-3521, www.velauno.com [1]) is the go-to place for all things surfing, including kite-surfing and windsurfing. They sell and rent equipment and offer lessons for adults and children, including weeklong summer camps for windsurfers.
Costazul (264 Calle San Francisco, Old San Juan, 787/722-0991 or 787/724-8085, fax 787/725-1097, sferco [at] caribe [dot] net, Mon.–Sat. 9 a.m.–7 p.m.) sells surfboards and related equipment.
But if you need instruction, Caribbean Surf School (787/637-8363, www.caribbeansurfpr.com [2]) offers daily and weekly lessons with avid surfers and certified lifeguards at various locations. The school has a 3:1 student-to-instructor ratio, as well as individual instruction.
Links:
[1] http://www.velauno.com
[2] http://www.caribbeansurfpr.com