Generally, several suppliers should work. However, you have to take care when planning to have DIFFERENT pipette types in the labs. They can vary greatly with regard to which tips are compatible. It should be best to have only one sort of tips around. If you already pipettes the new ones should fit as well.

I can only recommend that you ask a few different suppliers to lend you a range of models to test in-house, then run some gravimetric calibrations and/or use them for your preferred application.  If you're buying in a large batch of pipettes, you may be able to get a competitive discount.  I've happily used Gilson pipettes throughout my working life, but I think this is because historically there were few alternative brands available (and they last a long time, so why spend the money to replace them?).  However, there are lots of other newer brands which may be lighter and have ergonomic designs which are kinder to your hands – worth a look at least.  As the other responder mentions, tip/pipette compatibility is an important factor, especially if you have the option to buy good quality, universal tips at a budget price. 

Is there some reason you don't just get more of the same brand of pipets you have in the lab. There is something useful to being standardized across the lab so that you know tips fit well and you can share a stock of spare parts.