It can be done. My late mother in law used to sew all of the dalmatics for the altar boys in her parish and my wife made a pair of vestments once for my father. It is difficult, but if you hang in there it can be done. There are a number of sites online where you can obtain the fabric and trim, even local fabric stores may have stock that might be appropriate or have books to order from. Good luck!
FYI, my wife used the old vestments for more than a pattern, she took off the metallic galloon and emblem as well as the clasps and reused them in the new set. The remaining material was later burned with other old liturgical items by my father at the Church.
