Welcome to Scubaboard!
Good deals on used gear can be found on ebay if you know what you are looking for. Be sure you can get it serviced locally 1st. I've had good luck on ebay, but knew exactly what I wanted, knew what price was "good", and did my own servicing.
Buying "used" can be a great deal, just be very sure you know what you are getting (for example, I would avoid buying used Dacor regs since the brand is now out of business and it will get harder and harder to find rebuild parts).
LeisurePro.com , ScubaToys.com , DiveSports.com , and other online retailers often have really good deals on new equipment, so it is worthwhile checking out the new gear as well. ZeagleExpress.com and others are selling their remaining stock of just-discontinued Zeagle Flathead VI regulators for about $350 (this was an excellent >$500 regulator last year, it was replaced by the Flathead 7 this year).
There are deals out there if you shop around.
Which manufacturer? All the major players make great regulators. Aqualung, Scubapro, Oceanic, Mares, Zeagle, Atomic, DiveRite, Apex... the list goes on. Any mid-range regulator from any of these brands will be reliable and would make you very happy.
Some dive shops will sell their used rental regulators for a good price. This is worth checking into as well. Some dealers may also give you a price break if you are buying 3 sets of regulators, another thing to look into.
Best wishes.