i gotta agree with the others. vtg seems to have the best selection of single's rates.
i'm a solo traveler too--some other stuff i've found: the rccl brands: royal caribbean, azamara, and celebrity, seem like the cut some slack for single cruisers.
but the carnival brands, i.e. princess, hal, cunard, etc. they might have specials, bu they don't much 'hand out' solo fares--unless y'all are willing to pay the whole 200%, which i've done a couple times for a good deal.
other than that, there's british company that has some good deals----usually includes r/t airfare from london and a cruise--but they have singles pricing.
oh yeah, their prices are in british sterling so i usually multiply by 1.3 (close enough)
oh yeah, check the 90 day ticker on vtg too. ever once in a while--you'll find it's less expensive to pay 200% of the double occupancy fare (without taxes/fees for the non-existent 2nd person--than the solo traveler fare.
and, if you've got a ton of time on your hands, you can look for the anomalies: the deals that aren't supposed to be there. i finally came across one of those: 3 day california coastal on princess: $28 solo fare + $70 taxes/fees. anyhow ... good luck.