That's so funny... I was just about to ask the same question. I bought a bunch a couple of years ago and believe me a little goes a long way. It's been so long I can't remember where I got it so I was gonna ask where anyone else gets it.
I also used the search feature in SMF and learned that some people order it off of eBay. I found a couple of possibilities there.
As for what kind... I think as long as it is undyed and not blended with other types of fibers you're ok. I prefer the raw, unbleached noil fibers but I've heard where some people simply buy silk garments at a thrift store and chop those up into their lye.
Mine is from TheScentWorks (the raw, unbleached noil type). I agree- a little goes a looooong way. I'm still whittling away at my first and only bag that I bought over 6 years ago!
Oh Jeez!
I just remembered that I still have my invoices from back then and I just found that I also got mine at Scent Works... in 2009! And I only ordered 1 Ounce!!
I'm so glad I looked... for some reason I had it in my head that I needed to order 8 ounces. I would probably have to take up knitting if I ordered that much.
I use silk in almost all my batches. I put in literally a pinch in a 5# recipe. i incorporate it in my lye/water mixture. I t can take a few minutes to dissolve but I found the more you can separate it and not let it clump up, the quicker it dissolves.
4oz's doesn't sound like a lot but silk is very light. I bought a 1/4lb and its a lg freezer bag stuffed solid. http://www.soap-making-resource.com/tussah-silk.html
1/2 oz of silk will probably be much more than you think.
I pre-mix my lye (a.k.a. master batching) so I can't give you an exact measurement per batch of soap. I basically use a small handful of silk (probably just a couple of grams really) to 4 pounds of lye plus 4 pounds of liquid which makes about a gallon of 50/50 lye. This is then measured out and cut down further with more liquid when I make soap.
I start by shredding the silk into very wispy strands and then soaking it well for 10 minutes or so in my liquid. Then I slowly add the lye, stirring until everything is dissolved.