Most ATX to AT converters don't provide -5V, not unless the ATX power supply also provides it (which only those about 15 years old or older do). To get around that, some converters come with their own voltage converter, which gets -5V from one of the other rails (usually -12V), but such converters are hard to find and quite a bit more expensive. So no need to concern yourself with them unless you absolutely know you need the -5V rail. Here's a topic with a list of cards that require it, so unless you want to use one of those, just get a regular converter and use any spare modern ATX power supply you might have lying around.
Do a search on ebay, or wherever you prefer, for "atx to at p8 p9" and you should find plenty of options. Make sure to get one that comes with leads to connect a power switch. Some even come with switches, that is probably the simplest option. Something like this would be ideal. To use it, connect the black end (the ATX part) to your ATX power supply and the two white connectors (AT P8 and P9) to your motherboard, making sure the black wires on both parts end up adjacent when you do.