How to convert +5V power source to +/-5V power source
So you have +5V available. You can use a linear regulator to drop that to +4V and a linear regulator to drop +5 to 2.5V.
A DC-DC with isolated output will give you -5V or -3.3V, which you can then similarly regulate to -2.5V and -1V.
That's one possible overall topology. However the devil is in the details- the DC-DC will introduce noise into the +5V supply (on top of whatever other digital noise may be there) so you will probably want to add various filters to the input and output of the DC and to the inputs and outputs of the linear regulators, and note carefully the PSRR at various frequencies of the linear regulators. Usually they do better when they're not micropower regulators and when they have a bit of headroom to work with.