GSM TriAngulation Equipment wanted

Hello all,

I am in the business of selling & installing GPS tracking devices for fleet tracking and personal tracking. I am in a small country, and the Justice Dept has asked me to come up with a affordable plan or method to locate or triangulate GSM phones of which the IMEI number is known. This is needed for criminal investigations.

Does anybody know of a solution or vendor who can offer software/hardware sale to be able to do this even on a limited scale. I am not talking about a full fledged state-of-the-art triangulation system. Just a simple thing to be able to get some clue of the location of a GSM phone to be able to generate investigative leads, provided of course the IMEI number or phone number or both is known.
The GSM Service Provider would be involved in this by providing not only data and support, but also IMEI numbers if the phone number is known, or the phone number if the IMEI number is known.

Robert