In order of decreasing goodness.
Plan a: google for the key words i.e. 'man dies soul scattered cebes' and find it in a google book (which you can reference as if you read the paper edition) or find someone else quoting it in a piece of academic writing and either quote it indirectly or just rip off their reference list... That last option is a bit fraudulent but undetectable if the person you're ripping off did their referencing properly.
Plan b: quote the source you've got as a web page and put 'no page number'
academics are power users and will probably control F it on a web page if they're bothered. (if it being marked by a philosopher they probably remember what plato said about souls anyway)
Thanks folks. I was just using Plato as an example - that wasn't what I was trying to reference (it's widely available in book form). But I found a Google book (with page numbers - woohoo) for the text I was quoting, so it's fine. For future reference, I now know about 'n.p', learn something new everyday here .