Where can our parish purchase low-gluten hosts and mustum that has been approved by the USCCB?
From the Committee on Divine Worship Newsletter, October 2012 (updated August 18, 2015) Copyright 2012-2015, United States Conference of Catholic Bishops:

Celiac disease is an immune reaction to eating gluten, a protein found in wheat, barley, and rye. For those with the disease, eating gluten triggers an immune response in the small intestine. Over time, this immune reaction damages the small intestine's lining and hinders absorption of some nutrients. The intestinal damage often causes stomach pain, diarrhea, and weight loss, and can lead to serious complications. A Mayo Clinic-led analysis published in 2012 estimates that roughly 1.8 million Americans have the disease, but around 1.4 million of them are unaware that they have it.

Given the serious health risk for those suffering gluten intolerance, it is important for pastors and other Church leaders not only to be aware of the reality, but prepared to address the situation of Catholics with celiac disease who come to parishes and seek to receive Holy Communion in a safe, sensitive, and compassionate manner.

The most recent Church teaching on the use of mustum and low-gluten hosts at Mass remains the letter from then-Cardinal Joseph Ratzinger. . . on July 24, 2003 (Prot. n. 89/78-17498), which was addressed to the Presidents of Conferences of Bishops. In that letter, pastors and the faithful are reminded that for bread to be valid matter for the Eucharist, it must be made solely of wheat, contain enough gluten to effect the confection of bread, be free of foreign materials, and unaffected by any preparation or baking methods which would alter its nature. The amount of gluten necessary for validity in such bread is not determined by minimum percentage or weight, though hosts which have no gluten are considered invalid matter for Mass. (In the Roman Rite, the bread prepared for the Eucharist must also be unleavened.)

Mustum is defined as grape juice in which fermentation has begun, but has been suspended with the result that its alcohol content (usually less than 1.0%) does not reach the levels found in most table wines. It should not contain additives and may be stored through freezing or other means. The process used for the suspension of fermentation must not alter the nature of the juice in any way. The amount of alcohol needed for validity in mustum is not determined by a minimum percentage or weight. Pasteurized grape juice in which all alcohol has been evaporated through high temperature preparations is invalid matter for Mass. In the United States, it is forbidden to sell wine without the addition of sulfates as preservatives. The Church has determined that the very small amount of sulfates is acceptable and does not make the matter invalid.

The lay faithful who are not able to receive Holy Communion at all under the species of bread, even of low-gluten hosts, may receive Holy Communion under the species of wine only, regardless of whether the Precious Blood is offered to the rest of the faithful present at a given celebration of Mass.

Permission for priests, deacons, or the lay faithful without distinction to use mustum or low-gluten hosts is within the competence of the diocesan Bishop. The authority to permit the lay faithful to use mustum and low-gluten hosts in the reception of Holy Communion may be delegated to pastors under canon 137 §1 of the Code of Canon Law. Medical certification of a condition justifying the use of mustum or low-gluten hosts for Holy Communion is not required. Such permission, once granted, stands for as long as the condition persists which occasioned the request for the original permission.

As a best practice, it is recommended that individuals with gluten and/or alcohol intolerance arrange through their parish the purchase any low-gluten hosts or mustum. This facilitates the oversight and good stewardship of the pastor who is responsible as mentioned above. It also "normalizes" the practice for the communicant, as well as keeping the purchase of liturgical supplies together in the parish budget.

It is also worth recalling that, through the doctrine of concomitance, the Church teaches that under either species of bread or wine, the whole Christ is received (cf. General Instruction of the Roman Missal, no. 282; Catechism of the Catholic Church, no. 1390). Thus, the faithful may be confident in the Real Presence of Christ in the Eucharist they receive, even under only one or the other species.

The following distributors are approved by the Secretariat of Divine Worship:

Benedictine Sisters of Perpetual Adoration
Parish Crossroads

GlutenFreeHosts.com Inc.

Mont La Salle Altar Wines
Monks Wine & Candles