Regulated, variable power supplies are available from eBay, DX.com, Aliexpress and other vendors. Here is one, pictured on the left.
Voltage is adjusted by turning a screw and normally left at the desired setting. You’ll need a voltage display or a multimeter to measure voltage.
It is recommended the power supply has a watt rating at least 20% higher than required for your tube.
There are also professional power supplies for sale on eBay. These can be expensive but will probably last a lifetime.