Roman Catholic altar breads are available from the Benedictine Sisters of Perpetual Adoration, but some congregations prefer locally-made bread. Below are links to Web resources about altar bread in a variety of Christian assemblies.
"A moderate mixing does not alter the species, because that little is as it were absorbed by the greater. Consequently, then, if a small quantity of another grain be mixed with a much greater quantity of wheat, bread may be made therefrom so as to be the proper matter of this sacrament; but if the mixing be notable, for instance, half and half; or nearly so, then such mixing alters the species; consequently, bread made therefrom will not be the proper matter of this sacrament."
-- St. Thomas Aquinas, Summa Theologica, pars III, "On the Species of the Holy Eucharist, Art. 3:74," online in Western Communion Hosts: Some Recipes and Some Thoughts with other excellent resources on this topic from Prosphora.org, "devoted exclusively to Orthodox Christian Holy Bread."
E. Y. writes (Fri, 28 Mar 1997 21:31:37 -0600):
"The limits for valid matter are quite clearly stated in Pope St Pius V's Bull "De Defectibus" of 1572, and it states that any material other than wheat, water, salt (and yeast for the Eastern Rites) will invalidate the material."
Reflections on Eucharistic Bread and Wine:
"In a 1929 instruction, the Sacred Congregation for the Discipline of the Sacraments taught that bread made of any substance other than wheat is invalid matter, as is bread to which has been added such a great quantity [emphasis added] of another substance that it can no longer be considered wheat bread in the common estimation. ...The General Instruction of the Roman Missal #283 stipulates that the bread for the Eucharist should have the appearance of real food and be made in such a way that the priest is able to break it into parts and distribute them to at least some of the faithful. Subsequent instructions from the Apostolic See have attempted to clarify the meaning of this law by indicating that the "appearance" of bread applies to its color, taste, and thickness rather than to its shape. From The Code of Canon Law: A Text and Commentary (Commissioned by the Canon Law Society of America, page 657)."
General Instruction on the Roman Missal 2000:
320. Panis ad Eucharistiam celebrandam debet esse mere triticeus, recenter confectus, et secundum antiquam Ecclesiae latinae traditionem, azymus. (Institutio Generalis Missalis Romani. Caput VI: De Iis quae Ad Missae Celebrationem Requiruntur, I. De Pane Et Vino Ad Eucharistiam Celebrandam).
320. The bread for celebrating the Eucharist must be made only from wheat, must be recently baked, and, according to the ancient tradition of the Latin Church, must be unleavened. (Chapter VI: The Requisites for the Celebration of Mass, I. The Bread and Wine for Celebating the Eucharist).
"Altar Breads" in The Catholic Encyclopedia (1907-12).
Communion Bread Recipe (very-special-recipes.com)
Laboratorio Pane di Vita, Pisa. Altar Bread Production and Distribution (in Italian with drawings).
Lift Up Your Hearts. Sites Devoted to Worship & Liturgy (a comprehensive liturgical resource).
"Pennsylvania Altar Bread." (breadnet.net)
Prosphora (Orthodox Christian Holy Bread)
Our purpose is to help Orthodox Christians ... bake prosphora, and to help everyone learn about our rich traditions.
Saint Jude's Communion Bread. (Episcopal Church, Cupertino, CA).
"In some parishes it takes more faith to believe that the Holy Communion
wafer offered to the faithful is really bread than that it is truly the Body
and Blood of Our Lord Jesus Christ" (Catholic saying).
Rev. 16-Mar-2006 / archive.osb.org/liturgy/altarbread.html