Manuscripts from the 50's and early 60's are not commonly on the market, and will likely be very expensive if you do find them. 10 years ago you could still pick up 60's manuscripts, but I haven't seen any around in a while.
The letters from that period are easier to find than the manuscripts, I assume mainly because he didn't keep carbon copies of his poems, and he wasn't yet famous enough for publishers to preserve the manuscripts they way they did after he became more well known.