One form of silk I use is 60/2 silk yarn, about the diameter of two single strands of DMC floss. I buy mine on Ebay from a factory in India. Their Ebay store is called Silks Unique Traditional. The quality and price are excellent, the postage reasonable, and the factory rep speaks fluent English. 100-gram hanks of 60-2 silk are slippery and tangly and difficult to handle unless you have winding equipment (a swift, e.g.) and experience. The company is now offering this size on spools, which will be much easier to wind skeins from.
A drawback of 60/2 silk is that it is less resistant to abrasion than strands of fine floss. I use these stitching rules with ALL forms of silk, but they are particularly important with 60/2: work with shorter strands and use a larger needle which will open a wider hole in the linen for the silk to pass through. Cross-stitch expert Susan Greening Davis recommends a 22 or 24 size needle, and adds that it'll be like using a pickax for the task until you get accustomed to it. I use a 22.
A second type of silk I use is six-strand floss. It's hard to find. My only source for undyed high-quality six-strand floss in small quantities is Treenway Silks in Canada. They sell a 100 gram (3.5 oz.) skein for $23.80 (Canadian dollars). Postage from Canada is steep, so if you buy two or three skeins, the ultimate cost per dyed small skein will be lower.