I am glad I’m not the only one searching for strictly Advent music. My search has led me to a few CDs.
The first a great CD called “Advent Carols from St. John’s” recorded by the St. John’s College Choir at Cambridge. It includes lessons and carols, but if you want it with only the music and not the readings, you could simply burn a new copy with those removed. They sing some of my favourite Advent carols, including Adam Lay Ybounden, A Spotless Rose, There is No Rose, a nice setting of Ave Maria, the O Antiphons, which give root to O Come, O Come Emmanuel, and more.
The Sixteen & Harry Christophers also have a recording called “A Traditional Christmas Carol Collection, Vol. II” that is primarily Advent music, as opposed to Volume I which is primarily Christmas music. It is recorded beautifully and I highly recommend it.
Finally, there are a number of CDs which work their way from Advent to Christmas. The first half of many of these can be played until the third or fourth weeks of Advent and then transition into the entire CD. One beautiful example is “Carols from Advent to Christmas” by the Marlborough College Chapel Choir. Their version of ‘Jesus Christ the Apple Tree’ is particularly gorgeous.
I hope that helps! Please respond if you find something else that’s great.