Converters for PS/2 to USB keyboards are plentiful, but you will need to be vigilant in regards to the type you need. Both passive adapters and active converters exist, with passive adapters simply being a pin swap between the two types of connectors for more modern PS/2 keyboards that also support USB protocol internally. Your '80s Model M, for example, cannot do this, so what you need is an active converter. As a rule of thumb, active converters are specifically labelled as such, sometimes described as having a USB integrated controller (IC), feature a small box in the middle of the cable, and more often than not include both keyboard and mouse PS/2 ports together. Exceptions to this exist, however, so be sure to check out reviews and ask the manufacturer to clarify.