The Multi-Category Search at CPDL is extremely flexible, and with a little practice can get you precisely what you want (of course, it only lists music that is in the public domain and is available for download at CPDL, but that is still a vast amount of music).
http://www.cpdl.org/wiki/index.php/Special:RunQuery/Multi-Category_Search

The liturgical planning pages at CanticaNova would be helpful as well.
http://canticanova.com/pln_main.htm

Last I checked, his website (as listed in CPDL, anyway) had not been renewed, which is unfortunate.

CPDL does have a pretty good search engine. And if a text comes up that you like, but you aren't sold on the particular Motet, just try searching for that text to see if an editor perhaps just didn't categorize it in a way that made it show up in your original query.

Here's the link to Aristotle Esguerra's spreadsheet of polyphonic propers. I've updated Aristotle's list but it needs a proofread.

CPDL also has helpful pages for each of the Sundays and feasts of the year, which include appropriate music (with texts based on Magnificat antiphons, Gospel texts, etc.) as well as propers themselves.

You may find this forum discussion helpful as well.

If you really want to dig deep, this book in German discusses (with some transcriptions) otherwise little-known Renaissance cycles of polyphonic propers.

If anyone's thinking of compiling a "comprehensive" index of polyphonic propers (at least those by Renaissance composers), these resources may be helpful.

The Latin Motet: Indexes to Printed Collections, 1500-1600, ed. Harry B. Lincoln (Toronto: Institute of Medieval Music, 1993).
* Comprehensive (though not completely exhaustive) index of 16th century printed collections of motets by more than one composer (19,000 entries).
* Includes complete index of titles but no liturgical index.
* Spelling an other details of the entries are not completely reliable (see this review of the book).
* Can be searched online here.

Motet Database Catalogue Online
* Like the above resource, indexes motets from collections, but includes earlier prints (back to 1475) and manuscripts (over 50,000 entries, with many duplicates between prints and MSS). Projected to include motets from single-composer prints.
* Certain discrepancies in results based on secondary sources

Grove (Dictionary of) Music Online (subscription required) - entries on major composers include lists of individual motets

The Lipphardt book Jeffrey Quick refers to above

attached spreadsheet of propers keyed to the Liber usualis

The first two resouces index motets only from collections, not single composer prints. However, a number of composers published collections of Latin propers, most notably Isaac, Byrd, Senfl, Porta (introits), Palestrina (offertories) and Lassus (offertories). Lesser lights include Amon, Asola, Contino, Layolle, Lupus, and Zeilenski (early 17th cent.) and even the Lutheran Georg Rhau. There are also compilations by anonymous or various composers (Trent and Jena codices, indexed in the Motet Database; published collections discussed in the German book linked to above). Some later composers (F. X. Witt, J. J. Fux, 19th cent. Cecilians, Perosi) set considerable numbers of proper texts.

Some general cautions ("Propers for Dummies"):
* The full texts should be checked if at all possible:
- some proper texts share their first words with non-proper texts (e.g. Ave Maria . . . ). - - the same or similar texts may be assigned to various days, or to various parts of the mass on the same or different days
- the same text may or may not include alleluia(s) depending on whether they are assigned in Easter season or not
* On rare occasions the proper text(s) assigned to a day have changed since the 16th century, or their may be local variations (e.g. Requiem mass)
- conversely some propers serve more than one day.
Of course the 1970 missal changed large number of proper texts; often the three year cycle will dictate different texts in different years.
* Certain texts correspond not to particular days but to commons or votive masses, or may cover both a particular feast and a votive mass (esp. true of texts set by earlier composers).
* Sometimes a composer (most notably Byrd) will pair the gradual and alleluia (or pair other proper texts) together as two parts of a single piece, so they may not be listed as separate motets.
* In general, information obtained from secondary sources (e.g. all those listed above) should be double-checked with official or semi-official liturgical sources.