Other than those that show up on ebay I buy from Nick Balarini accordions in Dallas. (google him) He has been very honest and worked hard to satisfy the tough buyer negotiator that I am. I have purchased 1 used and 2 new ones from him and I certainly would contact him again when and if I ever had the need to buy another one.
Right now there is a FR-3s for sale right here locally on Craigslist for $1600. westernmass.craigslist.org/msg/3964173382.html I`m not sure of the tuning options on this box but I think it has them. Fr-3X models will run a few hundred more (used) . The slightly smaller FR-3/3X models are cheap and a great way to own a Roland cheap and determine if it is for you. . At 18-1/2 lbs you can play them all day long standing up.
If you find it`s not for you , you could get your dough back easily. If you do find it suits you well you could consider a FR-7X or the new 8X.
Nick is also able to get refurbished units which are as new and more affordable.
The FR3/3X and 7X and 8X models are professional models . Anything below them , you would be lacking keys, buttons and features, not to mention the quality of sound.
OK ,I just checked . The FR-3s is also fully tunable. 440 is factory default.
The 3 musette middle reeds can also be tuned for whatever +/- values you prefer such as French musette , Italian, French etc . Really nice !!
The Roland FR-7X user can tune his accordion from 415.6-440 .0-464.4
Also features scale tuning parameters for different cultures :Arabic 1 and 2, Indonesian, etc.
Just Major (classical western) just minor (pieces in minor keys) Pythagorean resoves ambiguity of 4ths and 5ths, mean tone, and much more.
Hope this helps.