I haven't ordered a will myself,but the advice,address and form(PA1S) are here(I hope):-
http://www.justice.gov.uk/courts/probate/copies-of-grants-willsThe records you are interested in are generally pre registration(1837) and while some may be on Family Search,detailed copies of parish registers(other than non-conformist) are generally on FindMyPast for which you will need to buy credits or take out a subscription.
A number of relevant addresses may be close to each other as follows(though inevitably there are many similar addresses,making it difficult to be sure)
The address in the probate,Cefn Coed is the same as the address in the 1901 census.There is such an address in Coed y Bryn,Llandysul.This is close to a property called Nant y Pobty(or Bobty),which is where Sarah James parents lived.Further,although it does not appear on current maps,there is also a Pen yr Allt close by.
Go to
www.old-maps.co.uk and use the search term- Blaenllan-it is not relevant but convenient.
Choose the Llandysul option and when the map loads.use the up arrow to find two of the above in Coed Y Bryn.
Return to Blaenllan and then click on the map between there and Rhippin du(upper left).Then choose the earliest 1:2500 map from the historical list on the right.This should give Pen yr Allt.
There is a property transcribed as Llynddy in the early censuses in this area but I haven't located it on the map.
Whether it helps or is purely of academic interest,there is a reference in Y Cymmrodor(at
www.archive.org) to a nephew of David Jenkin Morgan,called James Davies,the report being given by a Benjamin Williams who was the author of "Enwogion Ceredigion".
Regards
Roger