Here are a few yards that have them listed at a decent price....
2004 Third Brake Light Ford Explorer Sport Trac SLVR,4 DR,XLS,Privacy,Grey 5F0084 $25 249 Auto Parts USA-TX(Houston) Request_Quote 1-281-820-1111
2003 Third Brake Light Ford Explorer Sport Trac A 36084 $20 Ruppert's Pickup Parts USA-OK(Norman) E-mail 1-800-749-9287
2005 Third Brake Light Ford Explorer Sport Trac 6G0392 $25.95 Parts Unlimited, Inc. USA-TX(Pearland) E-mail 1-800-205-6375
2002 Third Brake Light Ford Explorer Sport Trac 4 DR,SPRTTRAC, A 507654 $30 Interstate Auto Center, Inc. USA-SD(Harrisburg) E-mail 1-800-296-1416