It is a little more complicated than just locating the inmate. You do not state whether you are serving a subpoena for a witness to appear, or an original summons attached to a newly filed civil complaint. A Maryland subpoena has no legal effect if served outside of Maryland. You would have to file a "miscellaneous" action in the other state, attaching the Maryland subpoena or summons, and having a new one issued from that state and served in compliance with that state's laws. If you are serving a summons on original service of process of a civil complaint, then the Maryland summons can be served without filing a new action in the other state, but it must be served in accordance with the local state's service rules.
The Bureau of Prisons has an inmate locator service at their website so finding him is no problem. If you need a federal inmate to testify in a federal matter, your attorney needs to file a motion for a writ of habeas corpus ad testificandum--in other words an order that the BOP produce him so that he can testify. I do not know whether the BOP will also produce inmates for state court proceedings, but I assume the procedure would be the same. If you are talking about serving him with notice of a lawsuit against him, the attorney just needs to use a process server in the state where the inmate is located.
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If you cannot locate the person at bop.gov, then the person may be in protective custody. Summoning that person for trial would require a motion and persuading the court to order it.
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