The correct bulb for your viewer depends on the make and model. The Pana View and Pana View II viewer (which I've used since the mid 1980s) use the 245 lamp. The viewer uses either two 1.5-volt AA cells in series (3.0 volts) or can be powered via the optional 120-volt AC to 3-volt power supply.
The 245 lamp is rated 2.45-volts and 0.5-Ampere. It has the 10 mm-diameter G3.5 (E10) screw-base.
These #245 lamps seem reasonably priced from these two sources:
https://www.lightbulbs.com/product/eiko-49703/?source=ConnexityCSE
https://www.bhphotovideo.com/c/product/40215-REG/Pana_Vue_DL_245_Lamp_for_Pana_Vue.html/?ap=y&gclid=EAIaIQobChMIhcPqv_HW4AIVKx-tBh3j_QeUEAQYASABEgIX2fD_BwE&lsft=BI:514&smp=Y
This seems to indicate that the lamps are being operated at higher-than-expected voltage. Perhaps the maker chose the 245 bulb for greater brightness and higher color temperature. A Pana View will work with a bulb intended for 3 volts, but will be somewhat dimmer and the color temperature will be a bit lower as well.
There are, or were, a range of different E10 screw-base lamps. Recall that flashlights are, or were, made in a variety of configurations. Those that used individual cells were made in various lengths to accommodate two or more cells in series. The voltage = 1.5 times the number of cells, The bulbs were chosen to accommodate the voltage of the cell stack, thus the bulbs were made in a variety of voltages, likewise for bayonet-base flashlight bulbs.
I’m not familiar with a 0.5-volt lamp for a slide viewer. What make and model viewer do you have? I wondered if you might have confused the ampere rating for the voltage.