The principal international catalogue of early music sources is RISM, Repertoire International des Sources Musicales, which has a searchable database www.rism.info/
Lists of tablature sources produced by the project Souces Manuscrits en Tablature, listed by country and by city, can be found at w1.bnu.fr/smt/sommaire.htm
A list of ‘finding tools’ for sources of lute music – lists, catalogues and databases can be found at
www.cs.dartmouth.edu/~wbc/lute/Sources.html
Cecilia is an on-line guide to music collections in archives, libraries and museums in the UK and Ireland www.cecilia-uk.org/
Note: IMSLP can be searched for downloadable scores and facsimiles by instrument, eg Lute, For 2 Lutes, For 3 Lutes. Here is the link for lute duets: imslp.org/index.php?title=Category:For_2_lutes&transclude=Template:Catintro
Different catalogues of songs and ensemble literature are listed at the Lute Library: web.archive.org/web/20151226121511/www.lutelibrary.com/ensemble
English renaissance lute music 1530-1630, Julia Craig McFeeley's PhD thesis: www.cs.dartmouth.edu/~wbc/julia/
Peter Steur’s database of music for the baroque lute: mss.slweiss.de/index.php?id=0&type=index&lang=eng
Francois-Pierre Goy and Andreas Schlegel’s Accords Nouveaux database: accordsnouveaux.ch
www.accordsnouveaux.ch/de/Abhandlung/Datenbank_PAN/Datenbank_PAN.html
Folias database, cataloguing all instances of La Folia: www.folias.nl/
Battle pieces, edited in successive issues of Il Liuto, Societa Italiana del Liuto: societadelliuto.it/?page_id=39
La Trobe University searchable early music database: www.lib.latrobe.edu.au/MMDB/
Gregorian chant homepage, Princeton University: music2.princeton.edu/chant_html/
Gregorian chant homepage (UK):[create http://music2.princeton.edu/chant_html/]
Global chant database: www.globalchant.org/
Diamm, Digital Image Archive of Medieval Music, www.diamm.ac.uk/
University of Florida motet catalogue: legacy.arts.ufl.edu/motet/default.asp
Viola da Gamba Society Thematic Index of Music for viols, including lyra viol tablatures:
www.vdgs.org.uk/thematic.html
Online database of 17th century ballads: ebba.english.ucsb.edu/
Anthologies of Musical Works in Print and Manuscript from the 15th-17th Centuries in the Library of Congress Music Division:
www.loc.gov/static/research-centers/performing-arts/documents/Anthologies-Final-Bibliography.pdf
and research guide guides.loc.gov/music-anthologies/introduction