Before I get into other legitimate options, I do want to mention the best option that could be available is Minnesota Twins catcher Joe Mauer. A .324 career hitter, he is hitting .326 this season, with a league leading .416 OBP. His 41 RBI is almost twice as much as the Mets catchers have accumulated this season. Though he adds instant credibility to the Mets lineup, he is signed through 2018 with the 8 year, $184 million contract he signed before the 2011 season. As much as the Mets will take on salary to improve their team, this is a contract they will not go near.
Nick Hundley was just sent down to AAA by the San Diego Padres after hitting just .166, 3, 22. He has more of an upside than Thole and the Mets could take advantage of his stock being down. Hernandez (.215, 4, 15) would be an upgrade, but I think the Mets should consider some other options before making this move. Geovanny Soto (.177, 6, 12) is off to the worst start of his career and the Cubs may look to move him before his stock drops even more than it has. He has averaged 17 HR over each of the past 4 seasons.
Kelly Shoppach (.269, 4, 12) may require more than a spare part or two to obtain from the Red Sox. The Red Sox have two catchers they can go with if Shoppach was traded, and the Mets are not the only team needing a catcher. Miguel Olivo would be my top choice, he hit 23 HR last season and has averaged 17 over the past 6 seasons. He is currently hitting .201, 7, 17 and its obvious the Mariners are going with Jesus Montero and John Jaso as their two future catchers.
Other catchers worth inquiring about are Detroit's Alex Avila (.242, 5, 42), Houston's Jason Castro (.254, 2, 20) and Los Angeles' Chris Iannetta (.197, 3, 9). Avila would require a lot being he is 25 and coming off a top season. Castro was a big time prospect before an injury derailed his 2011 season. He is also 25. Iannetta is injured but could be moved when healthy by the Angels.