I would just buy the whole cassette, an Ultegra 6800 11-32 is only $60 (us) so no big deal, but you would also need get sgs derailleur which is the medium/long cage derailleur to work with any cassette over 28 teeth. Another option would be to reduce the teeth up front depending on what you're running now, say you have a 53 tooth ring up front, dropping down to a 50 would help your climbing plus be cheaper than buying a derailleur and cassette.