Hi @emilyhines ,
Here is a link to just one [https://www.ebay.com.au/itm/Belt-for-Sony-WM-DX100-WM-AF48-WM-AF50-WM-AF57-WM-B26-WM-BF57-Walkman-/142181424199|supplier] of the part. It is not a recommendation to use them. it is shown to give an idea of the price. There are others online that may suit you better. Just search for ''Sony Walkman WM-AF48 belt'' to get results.
Here is a link to the ifixit [guide|30028] guide which may be of some help.