You may want to check on Ebay and on the official PSL site (google Texans PSL marketplace).
Despite people who complained over the last 5 years and said they were selling their PSL's, a surprisingly low number went up for sale.
I check every few months and other than lucking into PSL's on this site, that's your best bet(s).
2 front row 108 PSL's (30ish yardline behind Texans' bench) only $60k! (my wife said, wth, 2 years ago it was only $30k, we should bought them then and I pointed out the giant flaw in the plan, not having $30k).