Guys there is nowt difficult here...
You know your city (I assume) so go to the places that you know are derelict and see if you can get in!
Failing that use the search bar and type "sheffield", and hey presto a huge list of places. Some will be there, some will have been demolished... but a quick google will sort that out for you.
Even better, if you don't want to go to the places that have been trudged over for the past decade by every explorer, photo goon and pikey, have a drive / cycle / walk around the city and you'd be surprised what you will notice.
I'll even drop one in the mix for you to try... Eye Witness works is due to relocate either now or in the very near future (news is ambiguous) and as that is the last sizable unconverted cutler in the city, I'd say its one not to miss...