Most publishers have affordable anthologies of music for manuals only which feature late mediaeval, renaissance, and baroque pieces. These anthologies feature a wide variety of canzonas, organ chorals, organ versets for parts of the mass, and other pieces suitable for diverse needs.

Check into the catalogues of Concordia, Oxford, Cambridge, Novello, etc. For something affordable, but don't fail to have a look into the complete keyboard works of Tomkins, Bull, Byrd, Gibbons, Tallis, and The Mulliner Book, etc., in Novello's Musica Britanica series. I will enjoy imagining some of these being played at our sister (or daughter) parish at St Aelred's!

There are similar books from Boosey & Hawkes and Continental publishers such as Hinrichsen, et al., which feature short organ chorals, or canzonas, etc., and versets for singing alternatim - especially by the well known French composers. If one is not singing alternatim, one may cobble together two or three or more such pieces which work as prelude, offertory, or communion music of several 'movements'.

Titelouze(1563-1633)
Oeuvres complètes d'orgue

Tallis(1505-1585)
3 Organ Hymn Verses and 4 Antiphons

Attaingnant(1494-1551)
Deux Livres d'Orgue

Schlick(1455-1521)
Tabulaturen etlicher Lobgesang

also Scheidt(1587-1654) and Sweelinck(1562-1621). I love this echo fantasia SwWV 275

I do use the Montreal book frequently. Valuable resource for easy but fun repertoire.

Spanish Tientos, particularly those of Antonio de Cabezón, always seem to have real character to me. Love playing them.