If you're picturing a boob when you're squeezing a regular squishy stress reliever anyway, why not just cut out the illusion and make one actually shaped like a boob? This Instructables post shows you how in 12 easy steps, provided you have the prerequisites first (a woman that will let you use her boob as a mold). The creator says this:
Noticing that my breasts provide a source of comfort for my partner, I decided to take matters into my own hands and make him a personalised version of a classic erotic novelty: a squishy boob-shaped stress relief toy, modeled after one of my very own. Maybe now he'll leave me alone so I can get important things done.