A voltage regulator is designed to take a variable voltage in (say, 2-5v), and output a constant voltage (say, 3.3v). Now, voltage regulators are typically used to power a circuit, which means they will have a current output of a few hundred mA or more, generally speaking. In order to keep cost, size, etc down, the output tolerance on voltage regulators are (again, generally) a few 10s or 100s of mV.
For example, the RG71055 voltage regulator has a minimum output voltage of 5.2v, and a maximum of 5.8v, with a target output voltage of 5.5v, and can source 30mA. That's about a 5% voltage tolerance, assuming I number crunched correctly.
On the flip side, a voltage reference is designed to take a variable voltage, and deliver EXACTLY the rated output voltage. For example, the LT1790 can supply 5v with a tolerance of 0.1%, which is a 50x improvement over the RG71055. However, the LT1790 can only source 5mA max, which is 6x less than the RG71055. A voltage reference is used when you need to know that this line is exactly a certain voltage (in other words, really tight tolerances). On Digikey, you can get a voltage reference with 0.01% tolerance. With voltage regulators, you'd be lucky to get one with a 1% tolerance.
You are over-complicating this. The power rails for the circuit in your question are +2.5 volts and -2.5 volts and it uses a REF3125 to produce a REFP voltage that is precisely 2.5 volts above REFN. This makes REFP 0 volts but if REFN drifts to -2.6 volts, REFP must also track to -0.1 volts.
The internal reference voltage needed by the ADC is REFP-REFN hence, providing REFN is within the constraints dictated by the chip, it doesn't need to be tied to -2.5 volts. For instance if your power rails are 0 volts and +5 volts then you tie REFN to 0 volts and use a regular reference chip to produce (say) +2.5 volts.
In other words REFN doesn't need to be precisely set or even fairly constant but REFP needs to be a fixed value above REFN.
What is the standard way to get good quality bipolar reference voltage
in a circuit?
You don't need one - a common/semi_rough -2.5 volt rail (that also feeds the op-amps) is fine for REFN. You need REFP to be precisely set above REFN - that's all.
Best Answer
You could invert a +2.5 volt reference to -2.5 volts using an op-amp but I'd simply use a +2.5 volt shunt reference device like this one: -
And re-position is in the "negative circuit" like this: -
There are eighty-four +2.5 volt shunt references listed here on Mouser so I expect you'll be able to find one that is suitable in terms of accuracy and drift.
Note that the DS for the LM336 states on page 1: -
This applies to any shunt voltage reference of course.