what kind of attorney do i need to sue a sheriff department?

The first issue is not likely the hiring of a lawyer but that of a private investigator. Typically such short shrift by law enforcement is based upon poor investigation or cavalier attitudes toward same. Though they didnt ask certain people questions, it may have occured because, for example the alleged perpetrator, he was lawyered up or asserted 5th amendment privielege which prevented that from happeneing. In a bar fight situation, its often difficult to get people to cooperate with law enforcment etc. You would be well served to hire a PI to get the additional information in the form of recorded statemnts or affidavits, then if the info is helpful, possibly hire an attorney that has a freindl relationship with the local prosecutors to present it to them to see if they will reopen the case.