I've seen them offered OTR by the high end makers like Turnbull & Asser, Brioni, Kiton, etc....but at prices that would make it cheaper to go with a custom shirtmaker. My personal experience with silk dress shirts (or any silk garments) is that they wear hot in the summer and cold in the winter. If your body happens to run warm, as mine does, then they will get soaked when worn with a suit coat or sportcoat. Most of my formal shirts are made from either linen or cotton voile (with either a double front or pique bib) both having silk-like qualities but with the comfort and breathability of cotton.
Thanks. I was reading something about James Bond and it mentioned his preference was for heavy silk tuxedo shirts. So Turnbull & Asser makes sense.