Both of the large manufacturers Hayden and B&M are good.Bothhave models that fit Nissan/Infiniti's well.Sometimes they make more than one size, the bigger size you can get the better ussually.
They can be as cheap as $50 including all parts to install.
Or you can go to a shop and depending on type of shop and size of of cooler installed can be between $130 to $250
Yes its a good idea to put one in.
Even in winter they can be a good idea if you like to drive hard.Many have had fantastic results by combining frequent tranny flushes and tranny coolers to dramatically increase lonegevity of those trannies even when used very hard.
Autozone, NAPA, Pep Boys, and ushc generally stock either B&M or hayden coolers.
Fred...