All you need is a nicely tuned pistol for a sniper / DM sidearm. While I wouldn't necessarily go into a game with a pistol against AEGs, whenever I've ever had to draw my pistol in any game, it's always been used with excellent results. You just need a reliable, accurate pistol.
If you want a little more "oomph", then a machine pistol of some kind would work. There are some good recommendations above, but from my perspective, I would recommend the M11 over all others due to its small size and low weight.
The MP5 Stalker recommended works well. He's used it effectively for years and it's a fiable choice. The KSC / KWA / Umarex MP7 and MP9 are nice too and really fun gas guns to shoot, but they're actually quite heavy despite being mostly plastic. There is a drop leg holster available for the MP7 (not sure about MP9), but I think in a sniper role where you crawl and sneak around, this bulky, heavy thing on your thigh would be detrimental to your movement.
The reason I would choose the KWA M11A1 is its compact and lightweight frame. It can fit into most generic large-frame pistol holsters, has short mags available (long mag + holster = pain in the ass), and will provide a lot of firepower if required. I have a TF11 (low quality, but effective) and I've used it as a sidearm very effectively, although in my situation it was CQB, the only thing the fixed hopup TF11 is good for.