A lot of the replica guns like the SIG you're talking about are actually made by Umarex and are basically the same functionally.
You may be able to find one of them that's close enough to the SIG externally to work for you.
Here's an example.
http://www.pyramydair.com/s/m/Smith_Wesson_M_P_45_CO2_Pistol_Kit/3354
There are usually a few dead giveaways as to the maker. The "magazines" are all similar, for one thing, and some of them come with a couple of CO2 cartridges which often have Umarex stamped on them.