Visit the links page for a list of outdoor fabric sources with internet access. The old stand-by method is to use a seam glue, Seamgrip being the best product available. It is now possible for home sewers to use iron-on tape. For sources of tape, go to my links page and check out the fabric sources. You iron the tape directly onto your seam allowance, the goal being to heat the tape enough to bond the tape to the laminate without melting it.
Every iron has it's own temperature range, so experiment for the best results.
Too hot will melt everything, and too cool won't activate the adhesives for a good bond. On curves you can also use a combination of seam glue and tape. For more information on thread, visit the Tips page. One other option is to have a partially sewn garment professionally heat sealed.