question about bombers

What is a good shield and side arm for bombers?

Aegis is the cheapest shield you can get. Once you feel like investing in another shield, Grey Owlite and Crest of Almire are popular, since Elemental and Shadow enemies are tougher (on average) than Piercing enemies (Slimes can get tough with the right combinations, but Beasts are just a joke). If you have a sword side-arm, you may consider a Barbarous Thorn Shield for the Sword damage.

While Aegis is the cheapest shield by about 30k crowns, it's also a fine way to throw a perfectly good 1200 CE out the window on a shield you'll never use again as soon as you get literally anything else.
If you want to save crowns, make one (1) Volcanic Plate or an (1) Owlite Shield or a (1) Crest of Almire. Crest of Almire saves you about 500 CE so that's really the cheapest you can get apart from Ancient Plate.

What zeddy said. Aegis is pretty much the WORST of all the 5* shields. Even as a bomber, Royal Jelly or BTS (assuming you own even one sword) are better.

I usually equip a nitronome, a shivermist and a silversix as well as an owlite.
This question is asked all the time. For example, we recently answered it here: http://forums.spiralknights.com/en/node/71129.
Try this too: http://forums.spiralknights.com/en/node/70787.
And this: http://forums.spiralknights.com/en/node/71404.
Try also the "Bombing Guide" on the wiki.