Welcome David DeLong,
Thanks for sharing some of your spiritual journey. No you do not have to only buy Messianic Siddurs etc, it is perfectly ok to buy the traditional Orthodox Jewish ones.It is your intention that is Messianic. In fact I have never bought any Messianic Jewish Siddurim as they have been adapted for people who are usually protestant evangelical or charismatics in their theology. I prefer to use orthodox Jewish resources and use them with a Hebrew Catholic intention which is Messianic, Eucharistic, Marian and Josephine etc.
cheers br Gilbert Joseph