Just under search on the main page is a link to "refine search." That will take you to the full search page. Toward the bottom there will be a drop down menu that is defaulted to lowest fare. Click and change it to refundable. Your results will be displayed and you can find Y under the refundable tab between Choice fares and first.
If you're looking for B specifically, you can always use expertflyer or ITA matrix to find the availability and then call. Matrix will show you the price difference and whether a call is worth it.