In the UK, the HM prison service can help you locate an inmate who you do not know the whereabouts of. You can send them an e-mail at [email protected] make sure that in your e-mail you include all the details listed at: www.hmprisonservice.gov.uk/adviceandsupport/keepingintouch/l
A lot of the answers below indicate they are looking to find a US inmate. In the US all these requests are dealt with by the Federal Bureau of Prisons. You can see their website here: www.bop.gov/iloc2/LocateInmate.jsp
Their site will let you search by ID number, or by name to locate an inmate. Your request will be processed according the rules and regulations, so bear in mind that the inmate you are looking for may be asked to give consent for the disclosure of their whereabouts, since this is their personal data. Also, if you know which prison the inmate is in, you can contact the prison directly and ask them if you can come in to visit the inmate. More information about this for Suffolk can be found of their website here:
www.co.suffolk.ny.us/departments/sheriff/visiting.aspx
www.co.suffolk.ny.us/departments/sheriff/visiting.aspx