While playing Realm of the Mad God, I lost my weapon randomly and I can't get a new one. I tried looting bags I found but none of them were for wizard class.
Wander around the low level areas and look in bags. People leave low level weapons around all the time, just may take a bit to find one for your class.
You can probably still kill things with your spells, assuming you didn't toss out your spellbook as well. Assuming you have a decent spell, you can probably kill things of a decent level to try and get a better staff than you might find on the newbie coasts, but again with the randomization it could take a little while.
That's kind of how this game works. You are going to lose a lot more than just your weapon, pretty often. My advice would to just be loot everything you can that relates to you (even duplicates) and store them in the back at the nexus. So you have it again for repeated runs.
to answer your question there is no way to get your default weapon back without starting a new character... Or a kind soul gives you a weapon.