Voltage buffer, Operational amplifiers
If you need high input impedance (G ohms or nA/pA bias current) you can simply use a higher voltage op-amp (which will generally require a high supply voltage. For example, an LT1006 with a +40V supply voltage (it needs to be a couple volts higher than the maximum input voltage). Note that the maximum differential input voltage is 30V so it would be wise to add some series resistance and put back-to-back diodes across the inputs. Not all op-amps may require that. 

Otherwise, you may be able to use a high impedance divider and buffer that with an op-amp that has suitable characteristics (eg. low input bias current). It's probably not a problem to achieve a 10M input impedance- depending on all the many details such as temperature range, desired accuracy and so on.