If you can’t find a Rockwell I suggest an M-head. The M- is just a little larger than the original Rockwell. I have an M in storage and honestly you will probably like the M better dispite it not being R8. What taper do you have? There were three available B&S 7, Morse 2 (I think) and a B3. B&S 7 and Morse collets are still available new for reasonable prices.
You will prabably have to wait a long time before an original Rockwell vertical head becomes available and when it does it will cost as much as an entire mill. If you are intent on the Rockwell head I would suggest to follow the auction circuit in your area. They come up from time to time and that is the least costly way to buy. You may just find a cheap one in you area.
I follow AuctionZip and Govdeals.not so much eBay anymore.
Have you found a manual yet?