Gem? clad collecting revisited

I’m considering trying to build a clad set starting with 64-70 dimes, quarters, and halves. 
I searched prior posts for the pros and cons of acquiring by mint set, v.s. rolls, etc.
A couple of weeks ago, D posted a question about which clads were the rarest. Cladking mentioned the 64 dime and quarter, and 82-83s. He also suggested searching earlier posts for more information. I searched current and archived posts without much luck on specifics.
Could a couple of you knowledgeable clad collectors list the toughest coins to acquire by grade say thru 1999? Also, does anyone know the titles of the clad collecting resource books mentioned? Thanks in advance.

I searched prior posts for the pros and cons of acquiring by mint set, v.s. rolls, etc.
A couple of weeks ago, D posted a question about which clads were the rarest. Cladking mentioned the 64 dime and quarter, and 82-83s. He also suggested searching earlier posts for more information. I searched current and archived posts without much luck on specifics.

Could a couple of you knowledgeable clad collectors list the toughest coins to acquire by grade say thru 1999? Also, does anyone know the titles of the clad collecting resource books mentioned? Thanks in advance.
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Comments
for a few 1965 rolls and lot of '76 and states quarters rolls. If you can locate or-
iginal rolls there are very unlikely to be any coins which are choice or gem espec-
ially for the earlier clads. Other denominations are available but quality is often
a problem with these also. Mint set coins are generally well struck but can be even
more marked up than the roll coins. These are almost always struck by new dies
and are struck on the old vertical single presses.
The toughest quarters tend to be those which are toughest in mint sets.
In superb gem that means the '69, '82-P, '83-P, '83-D, '71, '74, '82-D, '77, '78.
If you consider marks then add in the '88-D, '87-D and a few other coins of this era.
In gem the toughest are '82-P, '83-P, '69, '71, '83-D, '74 and '77
Choice coins are toughest for ('82-'83), '71, '69, (several '80's and '90's), '69-D.
Original MS-60's are very elusive for the '68, '69, '67, '70-D, '71, and the '82/ '83 issues.
Clad dime rolls are tough for '69, '70. Gem dimes are fairly available for most dates
but several are tough in superb gem or with FB.
Not to put anyone on the spot, (but I am) who do you recommend as an online seller for clads?
Also, has anyone ever used Southern Coin, (meaning SouthernCoin.com, as opposed to great southern coin)?