I am very picky about my holsters and I only carry IWB. After my most recent purchase, I will not be purchasing a holster anywhere else. The company is called Fist Holsters. I know the website isn't the greatest, but don't let that deter you. There are some threads here on the forums about these holsters.
http://www.fist-inc.com/galeryholst.htm
They are totally top notch. They will make one custom to your 3" SP101. Because your SP101 has a spur on the hammer, I would definitely go with a holster that has a piece of protective leather that follows up the inner side of the gun (between the gun and your stomach). This will prevent the spur from digging into your gut. Even though I am a small/light guy, it is a must for me.
I have the #1 IWB, but my SP101 has a spur-less hammer. If I were you I would go with something like #21 or #44 (without the mag attachment of course
). You could probably choose any holster and just ask for the additional leather to protect your abdomen from the spur. Remember the holster colors (they give you eight choices) turn out really dark.
Let me know what you get and if you have any questions.