Although I did not buy them, I recall seeing white Irish linen pocket squares at the Nordstrom on Market Street in San Francisco about a year and a half to two years ago. My recollection is that they were Nordstrom's own brand, or possibly a name that I had never heard before. I have only silk squares, and only stumbled upon those while looking for others, so I don't remember much more.
The word is discreet. A "suggestion" of a pocket square is all that is needed. (Note Brian Williams on the nightly MSNBC news broadcast.) The classic linen square with the folded peaks works with everything. A plumped silk square (that accentuates or compliments or contrasts, not copies, the tie) should peek out of the pocket. Most good men's stores have brochures how to fold etc.... There is never a situation where a pocket square could not be used. (Well.... maybe if you are wearing a blazer with jeans.)
I agree with clarinet player. I wear them all the time. However, went to a funeral today, and didn't- guess that'd be a stretch. Thanks, guys, for the Ben Silver link, and the suggestion of Nordstrom. I stopped by a different one in The Bay Area today, and they had nice ones for around $15.00.