You've become yet another victim of not-so-good design choices by the bike manufacturers....I'll be glad to see the back of proprietary splined systems as will many others.
Splined BBs have always been a compromise too far in my mind. The diameter of the BB shell is obviously fixed and has to accommodate both the axle and the bearings. Good old square taper has a fairly small axle but massive bearings which is why the things last tens of thousands of miles. Plus they're mass produced and cost very little to replace when they do finally go. Thing is that thin axle is nowhere near tough enough for serious off road use. Riding square taper hard off road is extremely disconcerting due to the masses of flex and it eventually shears right off. So, an obvious 'solution' is to make the axle thicker. Works wonders for stiffness but now the bearings are reduced to a tiny space between the shell and axle. Result: The lifetime has reduced exponentially, even for smooth use, never find the battering and filth of off road. I'm not joking when I say I've seen brand new splined BBs disintegrate after a week of modest use. As to why they end up on hybrids presumably not meant for hard use we can only guess.
And of course, it wasn't enough to have one splined system, everyone had to get in on the act. ISIS was the most standardised and was if I recall created as an open alternative to Octalink. There's also powerspline, various truvativ offerings (holzfeller, hussefelt) and god knows how many others out there. And as you discovered, now that those systems are obsolete and mostly out of production, that you're left dead in the water when they pack in.
They're obsolete because even the manufacturers gave up trying to make remotely long lasting splined systems (I had one with 4 seperate bearings in it, still didn't last) and moved on to external cup types with the bearings outside the BB. I remember getting the first generation of Shimanos Saint external cranks back when I had a dirt-jump bike and they were a revelation. Massively stiff since the axle was the size of the BB shell and they actually lasted since the bearings were huge as well. They've now evolved and trickled down to the point where even quite low end road bikes and mtbs even now have them as standard. Problem solved - stiff, long lasting and reasonably cheap. So what's next for cranks? Oh yes, the BB30 press fit system, another solution in search of a problem.
Anyway, I would advise you to either upgrade to a modern external bearing system - expensive, but the best performance - or downgrade slightly to square taper. If I were you I wouldn't bother trying to keep the splined system going - it'll just keep causing more pain. Cheap option is to check out what the bike station has - have picked up a square taper crankset and BB for under a tenner there and then transfer the chainrings across.
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