I've been looking into the BC337 NPN transistor, and I saw in the hFE graph on page 3 of the datasheet, that the graph was at 1V from the collector to emitter. Is there any way to find out the hFE value for a different voltage like 5 or 10V? I've looked for a datasheet with a different voltage but have been unsuccessful.
The gain at higher VCE voltages will be similar, but not identical.
You don't need to know whether it's 1% or 10% higher, because gain can vary 300% from transistor to transistor, and also varies with collector current, and temperature. You need therefore to design your circuit to cope with a large gain variation. Compared to this large variation, VCE affects gain very little at all.
So given all this gain variation, why do people treat it as if it's constant? If the gain varies by a factor of 2, while the current varies from 100 nA to 100 mA, that is six orders of magnitude, then for an engineer, and for most use cases, that's constant.