I've seen it go for 20 PDs. It's an event item that can't obtained at the moment.
Edit: If you need it in a hurry, offering 30 PDs will probably get people jumping to sell you their blue-black stone.
"Two Kamui can be obtained by trading the following items in Claire's Deal 5 after defeating Shino:
Book of Katana 1
Book of Katana 2
Book of Katana 3
I do not know if Claire's Deal 5 is working. http://wiki.pioneer2.net/index.php?title=Claire's_Deal_5
This page (found by searching this thread) has some more details on Two Kamui.
Yeah, nobody likes digging through pages. The best way to find out the answer to your questions (other than asking other people, which can sometimes get lost) is to do a few searches. This is how I found out the answer to your questions.
To find the value of the item, you can type PC > Blue-Black Stone in the chatbox. Also, you can open up to the trade/sell forum group and search for black and then seeing what people offer.
To find out how to obtain an item, I'd first recommend checking to see if it drops in the drop tables. If it doesn't drop there, look up the item in the items wiki and see if something combines into it. If that's not the case, then a search in the Events page will reveal whether it is an event only item or not. If it is an event item, you should be able to see about when it's available to farm and what ID it will most likely drop for (not a for sure thing, as events can change). If those all fail, then you can try google or ask here.