The 245 will fit 226/220 holsters, the non railed versions, pretty much perfectly. It may or may not be a little loose in the railed versions, depends on the holster, but most likely a little loose. It will also probably fit most 229 holsters, but will be a little loose. I have all four and use the same holsters for the 220/226/245, and different holsters for my 229's.
The 245's dont have a rail, and a lot of 220's, 226's and 229's dont either. Most all the new SIG's do. If your getting the 245, I'd get a holster for the 220/226 made for a railed version. SIG's are addictive and the 245 wont be your last. This way as you acquire more, you can save money on holsters.
My favorites for my SIG's are the Blade Tech kydex IWB's, and lately the Milt Sparks Versa Max II's. The Blade Techs are about half the price as the Sparks and will probably out last them, especially if you sweat and wear it year round in a warm, damp location. I also have a couple of Desantis Speed Scabbards, which are comfortable and work well, but are not near as concealable as the IWB's.