Can someone recommend me a band that sounds like the fratellis?

pandora.com is great for finding something similar to what you like.

If it’s getting old, you should listen to something in a similar genre… but listening to a Fratellis rip-off probably isn’t the way forward.

Take a look at similar artists on the left hand side:
http://www.last.fm/music/The+Fratellis

Pandora is also a great site, as Kevbo suggested (provided you live in the US).

The Kooks
Arctic Monkeys
The Libertines
Hard-Fi
Dirty Pretty Things
Kaiser Chiefs

at least that’s what http://audiomap.tuneglue.net/ says

If you want “happy” then I’d go back in time and find other bands early albums before they get too serious. You could try Supergrass, and even further back Madness .

Check out music map for some similar artists.

I actually can recommend the Green Day Side project which is Foxboro Hot Tubs. Fun music and I can’t claim to be much of a GD fan.