Some cruise lines also offer lower single supplements on specific itineraries. Azamara comes to mind.

My advice is to "book what makes you happy" as there's no one else with you. Look for itineraries, lines, and ships which fit your interests rather than starting your search from solo cabins without additional supplement unless you're already happy with NCL and Royal mid to big ships. I do different activities when by myself than with others and desire a different level of fit, finish, layout, and service since I like to spend plenty of time in my cabin. Certain food quality is important to me, too, and I prefer smaller dining rooms.

Once you know what cruise fit and itinerary you'd like, look for a good deal even if you have to pay solo supplement. Solo supplement prices on inexpensive cruises can often be price competitive with studio cabins at premium times of the year.
cruiseplum.com can help search mass market cruiselines with solo pricing with different cabin categories.
vacationstogo.com shows single fares for a larger selection of cruise lines.