Rain waters no good, as it will contain pollutants washed down from the atmosphere (though probably wont make too much of a differenace) and rain doesn't contain any infusoria, you need pond water, fish tank water or water that has been sitting in a puddly outside for at least a few days and is clean.
What you could do for baby triops food is buy some 'NT Labs Brine Shrimp Food' from your lfs, and feed it 5 drops daily until your triops are larger.
Or you could mix some yeast into a paste and feed that to the triops.
But the best way is infusoria, since that way you cant over or under feed, which would pollute the water or starve the triops.