Where can someone find a copy of the liturgical texts for more recent feasts not yet published in the Liturgy of the Hours?
For some texts promulgated between the initial publication of the Liturgy of the Hours and the early 1990s, The Liturgy of the Hours (Supplement) is available from Catholic Book Publishing Corp.
In 2014, the Latin Church bishops of the United States approved a Supplement of the texts of newer feasts instituted in 2002 when the Missale Romanum, editio typica tertia was promulgated and the General Roman Calendar reformed. That fascicle awaits the confirmation of the Holy See before it can be published; those texts will also be included in the future Liturgy of the Hours, Second Edition.
Texts for the most recent celebrations established since 2011 (the publication of the Roman Missal, Third Edition) are available from the USCCB on the Liturgical Year & Calendar page.