Datastream (workstations 5 & 6 in Dewey Library). Do a Time Series Request for your closed-end fund under Data Category: Equities.
In WRDS, Select Compustat>North America>Securities Monthly. The Compustat Security Monthly (SECM) table has monthly data for prices and NAVs of closed-end funds.
For more information about closed-end funds, visit the SEC's website.