Thank you Black Lightning!There are several 7 speed 12-32 cassettes available at Universal Cycles: Dimension, Shimano, SRAM. SunRace has a 12-34. Your current rear derailleur might have enough capacity. Look for a model number, stamped on the back side of the inner parallelogram on most Shimano RDs.
https://www.universalcycles.com/shopping/product_details.php?id=114974&category=6976