PCMCIA cards are standardized, but there is more than one type. Older machines use a 16-bit standard, while more recent ones use a 32-bit standard known as "CardBus". I'm not sure which type your 120MHz laptop has, could be either one, given it's age. If you don't know for sure, your best bet is to find an older card that is 16-bit, as it will work in a 16- or 32-bit slot. My personal recomendation is a Cisco 340, since I know that they are compatible with the older hardware. I use one daily, and just bought a second one two days ago on eBay. It cost me $14.00 + 8.00 for shipping. If you search eBay for "Cisco 340" you should be able to find one for around that same price too.
--T