Try to go for a .COM if available, but some good alternatives include: .CO / .NET / .ORG
If you still can't find a domain you like, try adding "the" and dashes (-) to find even more availability.
Having trouble thinking of a domain name? Don't worry too much! Just do something related to the niche you want to go with. For example my ClickFunnels website is about "Funnels" so I went with FiveFunnel.com