Maureen..as a single cruiser myself...this is a hot button issue. You would think a few more cruise lines would wake up to the fact that we don't eat twice as much,etc..but I suppose their revenue projections are based on double occupancy.
Anyway, the ultra luxury lines all have more lenient policies with cruisers. For top categories they will charge 200%...but in lower categories they might charge 130% to 150%. Or on Radisson, in lower categories they just have a surcharge.
On the premium lines, Holland America has some lower single supplements.
On the mainstream lines, Costa allows singles to sail for a smaller supplement on a space available basis.
There are also older ships out their on various lines that have single cabins...such as the Norway and the QEII.
If you can travel last minute...there are some travel agents who specialize in seeking out cruises where the single supplement has been dropped all together to fill up the ship. But you are not going to get that kind of info without an agent.
Queen of Bermuda, New Years '89
Explorer of the Seas, 9/7/02
Queen Mary 2, 1/31/04