Here are some sources mentioning the Prague organ, that is, the organ used for kabbolas shabbos in the Meisels Synagogue in Prague, which was invoked as a precedent in the 19th century debate about organs in the synagogue.
The first is from Ezra Stiles, 18th century president of Yale, and a Hebraist. This passage from his diaries is from June of 1773 (published in 1902), and concerns some things that Rabbi Carigal, his guest from Hebron told him:
The second is from 1776, in Charles Burney's A General History of Music: From the Earliest Ages to the Present:
The third is from 1809, not long before the polemic over the organ would actually begin (1819). This is in Robert Adam's "The religious world displayed, or, a View of the Four Grand Systems of Religion, Judaism, Paganism, Christianity, and Mohammedism."
These are interesting, because R. Yechiel Goldhaber found that after 1719 there is no reference to the Prague organ, until the 1819 letter from the Prague Bet Din in which the existence of this organ was confirmed ( with the disclaimer that the music ceases a half hour before shabbos). Here are three sources later than 1719.
The early 18th century source he refers to is Schudt's Jüdische Merckwürdigkeiten. Actually, he doesn't provide all the references in this book. Here is one which he missed (Vol. IV pg. 155), although he refers to the same thing, namely a 1716 Yiddish newspaper called the Neue Zeitung which refers to one Reb Maier Mahler's building and playing of the organ, which had cost 400 gulden. According to this, this organ was in the Altneuschul: