Well, those 6 sources search the vast majority of scheduled airlines (basically anything that puts fares into a major GDS) so I'm not sure what else you expect some other source to find. You could also check Opodo (www.opodo.co.uk) but I'd expect results to be fairly similar. Also, be aware that sometimes European Expedia sites (for example, Expedia.de) will have fares that Expedia.com doesn't have or vice versa, due to sales restrictions - might want to check both.

Finally, whichbudget.com is a good reference of LCC's that might help you identify smaller carriers that don't participate in the major GDS, so you can look into booking directly.

Hope that helps.