"Porta viatico o porta relicario: es una especie de bolsita, de diversos materiales (tela, cuero, piel, plástico), que puede ir adornada con algún símbolo litúrgico. Lleva un cordón lago (para poder colgarse al cuello),y evitar que se caiga. Asi el ministro trasporta el viatico o el relicario.
Viático provisiones para el camino): con este nombre se indica el sacramento de la eucaristía dado a los moribundos, según las palabras del Señor: “Quien come mi carne y bebe mi sangre tiene vida eterna, y yo le resucitaré el último día” (Jn 6,54)."
http://es.catholic.net/op/articulos/58139/cat/912/el-signifi...

"The word pyx (Lat., pyxis, which transliterates the Greek, pyxis, box-wood receptacle, from pyxos, box-tree) was formerly applied in a wide and general sense to all vessels used to contain the Blessed Eucharist. [...] In later times however it has come about that the term pyx is limited in ordinary usage to that smaller vessel of gold, or silver-gilt, in which the Eucharist is commonly carried to the sick. Such vessels are sometimes made flat like a watch, sometimes mounted upon a little stand like a miniature ciborium."
http://www.newadvent.org/cathen/12588a.htm

https://en.wikipedia.org/wiki/Pyx

Compare these pages:
https://www.holyart.com/liturgical-accessories/pyxes-and-bur...
https://www.holyart.es/accesorios-para-la-liturgia/relicario...

You might need to specify what sort of pyx your mother-in-law wants, for example a pendant.

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Note added at 37 mins (2018-01-23 14:17:01 GMT)
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If it's the pouch she wants, it's called a burse.

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Note added at 1 hr (2018-01-23 14:39:36 GMT)
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In principle, a "porta viático" is often like a little round metal box, and that's a pyx, but the first definition I quoted in Spanish, as an "especie de bolsita", does correspond to a pyx burse. So as I say, check exactly what it is she wants. It might be something like this:
https://www.amazon.com/Pyx-burse-calfskin-cross-towel/dp/B00...