Y is easy. Just choose the Refundable toggle on the search results screen. B is a little difficult to select directly. Are you trying to choose fares eligible for 25% or 50% CoS bonuses? If so, choose MVP for the upgrade type to display Y or S fares, or MVP Gold for YSMB fares (lowest available, which usually is going to be M or B). Since those instant upgrade types are really just buying an eligible coach fare, it can also work for buying those higher fare buckets. Note that there's website logic using those options which will limit to one traveler for MVP upgrades and two travelers for MVP Gold upgrade types.

Other ways are to book through a travel agent, OTA, or the AS call center. Note that the last option will trigger a call center fee if it's a new booking. You can book the trip yourself on the website, then call in to upfare it for just the add collect. Use a tool like ITA Matrix to price it first, if you like.