i got loads of banshee sword arms to make my storm guardians and they dont fit too badly, just need to get rid of the shoulder pad on the side the arm is going. for shuriken pistols i got them from dire avenger exarchs. however not sure if GW sells parts separately now like they used to.
as for eldar chainswords, you can always see if there are any eldar weapon sprues going on ebay. these have 1 each of chainswords, powerswords, powerfists, fusion guns, flamers, lasguns and shuriken catapults, shuriken pistols, laspistols. you would also need an eldar arm sprue then too though.