Hi Nov, welcome from Murrieta! I agree with RikRong - your best bet is to attend some social gatherings in the San Diego area and get to know some local members. There aren't really any shops in the area that specialize in Xterras or offroading Nissans for that matter. You can catch some luck getting work done at shops like Off Road Warehouse or 4 Wheel Parts, but they're more for the jeep and full-size truck crowd. The truth is that Xterras are a very niche market with limited parts and modifications to be had. The people who can give you the best advice about these options can be found right here on this forum.
We have a meet and greet once a month (on the 3rd Thursday of the month) that takes place in the Mira Mesa or Poway area. Most recently, we've been meeting at In N Out in Poway. You can follow this thread for info on next month's meet up as it gets closer:
http://www.sccxterra.com/smf/index.php?topic=2699.240We also usually have a mod day once every couple of months somewhere in the area, which is another great place to meet people and get a good feel for what to expect when working on Xterras. Even if you're not the type of person who could see yourself working on your own vehicle, you'd be surprised how simple it can be, and with the guidance of some of the people on this forum, anything is possible.