Because the layout is non standard, modern caps aren't really a good option if it's one of those unique caps.
But if it does help there are 2 main options, SP/dcs and tai hao alps.
DCS is better but tai hao is more readily available.
if you are referring to alps AEK keycaps, than I don't think they would work well since they don't have the same profile I'm pretty sure.
You're best bet is probably buying replacement keycaps off of ebay, there are people who usually sell them so you could pick up the one you lost.
Hope this helps.