pickup for squier P bass
In part, it depends on how much you are willing to spend on a pickup. I took the plunge and installed a Bartolini 8S pickup on my MIM Fender Precision Bass. The Bartolini is advertised as a pickup that provides enhanced lows and low-mids, and it delivers. The pickup is a big improvement over the stock MIM pickup, which in comparison sounds harsh, thin, "barky" and weak in the lows. My tech also loves the Bartolini- he agrees that it is a superior pickup. It has a very "smooth", refined tone with strong yet clear lows. I wouldn't hesitate to my Fender MIM bass on a recording date, now that it has been equiped with this pickup.
Your local music store should be able to order one for you.
One issue to consider, however, is the cost of the Bartolini, relative to the value of your Squier P Bass. If you love the acoustic sound and neck feel of your Squier and intend to keep it forn the next several years, then perhaps it makes sense to spend money on something like a Bartolini.
A somewhat cheaper alternative is the Fender Original 1962 P Bass Pickup. It sells online new for $70. I do not own a bass equiped with this pickup, but the reviews are quite positive.