Can't Find Purchased Ringtones

I purchased a few ringtones last night,

On your computer or phone?
What country is your itunes account located?

When you connected your phone to itunes, did you File>Transfer purchases or did you just sync. If you did not transfer your purchases before you synced, that's why they were removed from your phone. Contact itunes support, explain what happened & ask if you can re-download to your phone. If they let you, make sure you transfer the purchases before you sync.

http://www.apple.com/support/itunes/

You should receive a popup regarding purchases on your phone that are not in your itunes library asking if you want to transfer or not. If you don't transfer, they will be wiped from your phone. The itunes content sync is one way: computer to phone. You cannot sync content from a device into your itunes library, it must be transferred.

Disable auto sync when an ipod/iphone is connected in iTunes. This will be under preferences, which if using windows is under the edit menu. Connect your phone after doing so. Right click the phone in the left device pane & select "Reset Warnings". You should get that popup from now on. Regardless, you now know you have to File>Transfer Purchases for any content purchased on your phone that is not in your itunes library. By disabling auto sync, you will be able to do so before you do sync.

Windows or OS X?

In itunes, Edit Menu, Preferences, it's either the general tab or devices.

You're welcome.