Most correctional facilities have a process for exactly this situation. Call or write the facility and explain that you have a legal document requiring the inmate's notarized signature. Ask to speak with the inmate's case manager or the facility's legal or administrative department. They can walk you through the procedure, which typically involves the inmate being brought to an administrative area where a notary public employed by or contracted with the facility witnesses and notarizes the signature.
This service exists because inmates regularly need to execute legal documents, from property transactions like yours to powers of attorney, estate matters, and court filings. Facilities are accustomed to facilitating it.
The second option is mailing the document to the inmate and having them seek out the notary service through the facility on their own initiative. Some facilities have notary services available through the law library or administrative office that inmates can access by request. The inmate signs in front of the notary, the document is notarized, and it is mailed back to you.
If you need a power of attorney rather than a direct signature, the same process applies. Draft the POA, send it to the inmate, they execute it in front of a notary at the facility, and it comes back to you. With a valid POA in hand, they would not need to sign the property documents at all since you would be authorized to sign on their behalf.