If you search on 'single cabins' in the keywords box you will find that this is an oft asked question. Those cruises that do offer single cabins are listed.
A few of the older ships have them. You can check which on the Berlitz guide on this site. The others that don'y have single cabins dont exclude single passengers they just charge you a supplement. They vary considerably but basically the better cabins (like outsides with balconies and suites etc, all carry 100% supplement. Some cruise companies do special singles cruises a few times per year where you can share with someone else to spread the cost. I've never tried that as sharing a cabin with a stranger doesn't appeal to me at all - I'd much rather pay the supplemen.