If it's the LMNA gene you're looking for, try the international mouse strain resource: http://www.findmice.org/summary?query=LMNA&states=ES+Cell&states=embryo&states=live&states=ovaries&states=sperm&_states=1&types=Any&_types=1&repositories=Any&_repositories=1&_mutations=on
If it's a different strain you're after, the website is still a good one to bookmark for the future.
As others have stated Jackson labs is a great place to go. You can probably obtain these mice from other researchers using them for free (just the cost of shipping), however, and should consider that option. Keep in mind the the LMNA -/- mice are not a good progeria model. A much better model is the ZMPSTE24 -/- mouse. In Hutchinson-Gilford progeria it is a mutation in LMNA that results in aberrant processing and accumulation of the farnesylated form of this protein. In the LMNA -/- mice they simply lack lamin a altogether. ZMPSTE24 is an enzyme involved in LMNA processing and loss of it results in accumulation of LMNA, more closely modeling the human disease.