Let me be more specific about my question's topic: in regards to praying Tehillim for individuals I am referring, here, to the recitation of Psalm 119 (the personal portion following the standard order of prescribed Davidic Psalms preceding it). *
Question itself: What should I have done for the name מִרְיָם seeing as it ends in a ם and that is not an available corresponding letter in Psalm 119? The way I dealt with it was to just repeat the verses for מ at the end of her name.
***At the shul where I usually attend, we do not say Tehillim during services, but recite/sing a short prayer for complete healing in the middle of Friday night services. So, the Tehillim order is fairly new to me, except for my brief exposure to it at a Chabad shul. Thanks. And may Hashem merit us all to have a full and speedy recovery from these winter illnesses!