I'm not sure how an IWB holster would fit (do you buy pants that are bigger than you would normally wear?).
It depends on how tight you wear your pants. I have some pants that fit comfortably without a holster, and I can wear a holstered 1911 pretty comfortably. I have other pants that are just a little bit tighter, but still comfortable, that make it a tight fit getting a holstered pistol in them.
I think the two best holster materials are either leather or kydex. Lots of guys like the combination of the two in holsters like the Tucker Answer (
http://www.tuckergunleather.com/the-answer-iwb/ ) or Crossbreed Supertuck (
http://www.crossbreedholsters.com/Home/tabid/36/Default.aspx ).
For leather you could try Milt Sparks. Their Summer Special/Summer Special II and Versa Max II are two of the industry standards.
http://www.miltsparks.com
For kydex, Garrett Industries has some very nice leather lined kydex.
http://garrettindustries.mybigcommerce.com
Another option is the Raven Concealment Phantom. It is a holster that can be worn either IWB or OWB just by changing the attachments. You can also adjust the ride height and cant, so it is very handy for the guy new to carrying so you can try out different options to see what works best for you. In the event you determine you just don't like IWB carry, you still have a perfectly good, concealable OWB holster.
http://www.ravenconcealment.com
I often see comments on forums about how hard it is for the heavy guys to conceal carry, but I think being a heavy guy is an advantage.
First you are probably larger than the thin guy, front to back, so you have more space to place and hide the pistol. The butt of the pistol is the hardest part of the gun to conceal, and it usually is orientated on the horizontal, so more front to back size is an advantage. The holsters with wide spaced loops , like the Crossbreed and Versa Max II, also work better for the big guys since they have that extra real-estate to place those holsters.
Second, you will generally have more "compressibility" of your body than the thin guy. For instance, a soft spot like a belly (or a well padded hip on heavy guy), can be compressed or moved easier, to allow the presence of a holster and gun, than a boney hip on a skinny guy.
Third, larger baggy clothes that aid in concealment generally look more appropriate on the larger person than on thin person.
