It will DEFINITELY be very difficult and yes, you will likely pay a small fortune - or at least double the normal going rate. I'd start searching this forum using SEVILLA (or SEVILLE) HOTEL as your keywords and start checking out the websites. Chances are, the ones with websites will be full first so you may have to start going down lists of those without websites and call them by phone until you find one with availability. Be sure to have all the normal questions ready to READ to them when you call. You can surely get their addresses on the internet and then can probably find them on an internet map as well and then choose the one with the best location and/or best price.

Start with this list of links and descriptions of hostels/hostales, pensions/pensiones, and youth-hostels/albergues juveniles in Sevilla which have websites: http://www.SpainHostels.com/links/Andalucia/Sevilla/Sevilla/ . DO IT NOW! Every day that passes means fewer rooms available.

We're having the same problem for Semana Santa in San Sebastián-Donostia. Nothing like waiting 'til the last minute, eh? (wasn't MY idea to wait this long )

Good luck! Saludos, MadridMan