Your best bet is to offer to share a cabin with similar age etc. i did that two years ago and it turned out wel.
I cruise with P&O, and can get reasonable single supplements of between 25 and 50 percent. However, as P&O only have a limited number of single occupancy cabins, it pays to book as early as possible, and these low supplements are not usually available for the spring/summer months. Generally this means cruising in October/November/December. I hope this is of some help.
I thought that Saga accomodated the single traveller - and I have heard that one line [Cunard??] offer cabin shares for single passengers - which sounds ghastly to me but may be a way of keeping costs down.
Fred Olsen offer single cabins of varying grades as well as cabin share.
Watch teletex, you can often sometimes get cruises with no single supplement. I managed 4 last year, this year 1 cruise with only £100 single supplement and in September I am off again without paying a single supplement to the Greek Islands with Monarch Classic Cruises which can be booked through a London Agent Jodie Stevens on 0845 330 0387. I have made so many friends cruising alone, it holds no horrors for me, in fact I actually prefer it. Good luc.