Quite a few of the later works are easily available in modern editions, but some aren't that easy to find, so here are a few links that might help:
The Poems of Ossian:
James MacPherson's "Ossian" (E-text Index)
"Kilmenny":
http://www.horrormasters.com/Text/a_211.pdf
(This site also has quite a few other poems and tales by Hogg in the supernatural vein.)
"The Wild Huntsman" (as translated by Scott) and "Lenore" (as translated by Rossetti):
http://www.horrormasters.com/Text/a_299.pdf Lenore
Thomas Moore's "The Ring":
The Complete Poems of Sir Thomas Moore eBook
His
The Epicurean and
Alciphron have been reprinted in a 2003 paperback edition by University Press of the Pacific, still available I believe -- though these are not supernatural tales themselves, they do have some very striking eerie passages.
"Sir Bertrand", complete with Ms. Aiken's introductory essay, is here:
Aikin, from Misc. Pieces in Prose (1773)
while the tale itself is available in several anthologies.