Finding and Downloading Voicebanks
There is no absolute area that voicebanks are uploaded to, but there are many sites you can find them.
People tend to post showcases of their voicebank on release so you can listen to how they sound before you download.
Most voicebanks have their own website and many downloads are hosted on BowlRoll.
Generally, the catch-all search terms for voicebank downloads are "UTAU Release" or "UTAU Newcomer" for English, and "UTAU音源配布" for Japanese.