Sadly, while Derick S. Thomson's
Gaelic Sources of Macpherson's 'Ossian' remains a watermark
in scholarship on this subject,
this publication is out of print,
and I can find absolutely NO second-hand, or any-hand, copies for sale online.
A lending library here has one circulating copy, which I have borrowed and will have to return soon enough.

I have not read every last page at this moment,
although I have begun reading a little at a time.
Mister Thomson's English prose is thankfully clear and direct;
knowing that this piece began as a 'tripos thesis,' I dreaded the kind of postgraduate jargon which makes some scholars sound as though their left brain has got a permanent warp through it.

In the Introduction to his own thesis,
Thomson names Alexander Cameron and J. F. Campbell, citing their 'transcripts'
of authentic Gaelic source material -- in the Gaelic, not translated.

Then, too, this book
has a detailed bibliography that identifies
all the published material used in writing the thesis.
It's not a large book at all,
but it is indeed a book of substance,
and one needs to be educated to understand the whole of it.