So I was searching around youtube, and I came across a video of a Sanshin with a Shamisen neck. The dou of the sanshin used in the video is the exact same design I have. So my question is, how can I get a Nagauta Shamisen neck to put on my Sanshin dou like in the video?
Video: https://www.youtube.com/watch?v=hcaPW-ZQNKg
Whoa! I’ve never seen that before! I think he must’ve already had a nagauta shamisen with a busted skin and simply removed the dou. Likewise, I think the easiest (and cheapest) way for you to get a shamisen sao would be to find a busted nagauta shamisen on ebay and take the sao from that. Getting a new sao separately will probably cost more.
Indeed, but you would have to worry about the square part of the sao that goes into the dou (I’m sure there’s a Japanese word for it) being a snug fit. I though that if I ever get a better shamisen I’d keep my current one around so just in case the kawa breaks I could swap out the dou, but then I read somewhere that each sao is fit for the specific dou that it is intended for and may vary.
Ah yes, no doubt some modifications will need to be made, angle-wise and shape of nakago (square part). But hey, it’s just wood. 
If it’s too big, you can file it down a bit. If it’s too small, you can add in a wood shim to tighten it up.
Anything is possible!
I have a genuine snakeskin dou (body) that I will sell for $65 plus shipping. I brought it back a year or two ago. I also have 2 or 3 Okinawa (not kuroki) sao (neck) for $20 each plus USPS. You can write me at photomojo@gmail.com. Or facebook: https://www.facebook.com/del.ihle