228.  Some 229, tho a few may be a tad large.

The P228 would probably be the closest in size to the P6, but IIRC the P228 is still about an 1/8" wider. Buying a holster actually molded for the P6 would be a better choice.

Raven Concealment, Galco, Milt Sparks, Alessi, and Crossbreed all make IWB holsters for the P6. I've been wanting to get an IWB for my P6 too but just haven't decided which one to try first.

My p6 shares the same galco holster with my  p228. Galco summer comfort.

It will fit in Sig 228 holsters quite easily.  My 228 rides in a P6 surplus and I know of two P6 owners who use 228 holsters and they're snug fits.

If you like kydex, look no further than Blade-Tech, Comp-Tac, or Palmetto Tactical Gear (Raven style, without the wait). I have all three. The Blade-Tech Nano is a great holster. My only issue with it is that I can't position it EXACTLY where I want it due to the soft loop positioning. The perfect spot would be right over a belt loop. The Comp-Tac C.T.A.C. is my reference IWB and it's what I carry my P225 or P6 in daily. Unfortunately, they stopped producing holsters for the P225. BUT my P228 C.T.A.C is a perfect fit. No rattle, smooth draw.