8 speed cassettes are not hard to get, any 8 speed Shimano mtb cassette will work with your bike and if the biggest cog exceeds 28 teeth and you have a short cage dérailleur you will need to swap that for a mid or long cage rear derailleur.

The mid length Ultegra rear dérailleur will work with 30-32 teeth quite nicely... a long cage mtb rear dérailleur would be the most economical option and offer very good performance with a mid level model.