Here are many different styles to choose from. I see in the list 6 inch and 12 inch fluorescent lights. One or two of those should be plenty for any size closet.
The new high output LED lights are good also and produce a bright light on very little electricity so your batteries can last a long time.
Here is a list to compare prices and features. Battery Operated Closet Lights - Home & Garden - Compare Prices, Reviews and Buy at Nextag - Price - Review