I don't know about the "+" part, but you can get the base device (as well as the uIEC) from here:
http://store.go4retro.com/quadportiec/
My friend Jim Peters makes these, but they are in very limited quantity. I have sent him an email to come here and comment whether he is making more.
Naturally, he's made one for me, and I love it. My favorite part is that you can switch out the uIEC if you're resetting the computer without resetting the uIEC. This is nice if you want to leave it in a disk image.
I don't know about the "+" part, but you can get the base device (as well as the uIEC) from here:
http://store.go4retro.com/quadportiec/
g.
.I really don't know what the PLUS refers too. I have never seen the "non" plus version. I was happy to see this because I already had the uIEC, so this was a much cheaper option over buying one of those mini drives you see on Ebay.