I haven't actually tried it, but a few years ago Redragon's MMO mouse was recommended to me by a former Square employee.
I have this one which is apparently programmable. I'll have to mess with it some time to see if I can just macro some weird key combination to fake having extra buttons.
I suggest Reddragon predition...I have got this mouse for over a year and no problem whatsoever. It comes with special program where you can assign 12 side buttons to any key you want. Also its half price of Razer. https://www.amazon.com/Redragon-M901-PERDITION-High-Precision-Programmable/dp/B00GU8W5AE
I use the Razer Naga 2014. The mechanical side buttons feel really nice to use. Razer's "Synapse" software works decently well with Mac, as you can bind any button to a multitude of functions. The only issue I've run into is that the OS-level mouse tracking seems to reset to 0 on reboot. Not a huge issue. I would definitely purchase this mouse again if mine were to ever go up (had it for about 11 months now with no issues.)
The Logitech G600 has decent reviews, although I didn't go with it because a) the side buttons are squishy, and b) the software only works in Windows, so if you want to rebind keys to something else, you're out of luck.
A cheaper option is the Redragon Perdition, which is a newer "MMO mouse." I couldn't verify, but I don't believe this one has any Mac software either. However, the buttons are still just numeric keys, so if you don't need anything crazy, it should be fine. You can also store the button profiles on the device, so if you have a Windows install, you can apply it there then use it in OS X.
There's also the UtechSmart Venus. Not bad reviews; fairly new; cheap. Like the two previous mice, I don't believe it has any Mac software, but the extra buttons will still function.
The mouse I use is this one
I highly recommend it for price, lightweight and it's macros you can set