It's tough to design a very low minimum current reference,
you end up with huge resistors and subject to all of the low
current nonidealities (beta rolloff, subthreshold slope variation,
leakage floor, etc.) your available devices may exhibit. There
are however a lot of new products in this space aiming at
the IoT and handhelds, all very power-draw sensitive. You
ought to broaden your search.
You also express hardly anything in the way of specs other
than a silly-wide range of acceptable reference voltage (and
no mention of accuracy required, across what temp range,
etc.).
It might be that a good zener (maybe stacked on top of a
diode, which is called a "temperature compensated zener" if
it's done on one discrete-diode die) with an acceptably low
knee current could do the job - 6.1 - 6.2V is the norm for
these, a post-divider could scale it -if- the reference voltage
load is low and varies little (like, bias at 40uA, throw 10uA
down the divider and 30uA at the zener).