Can you provide more details? What type of environment are you currently using?
Definitely more details required – what are you running, what are you protecting…….what size is your operation (to get an idea of the scope). For home usage, a router might do…. or an extra PC that isn’t in use. For larger networks, the possibilities are numerous. Here are some hardware solutions to maybe help get an idea of what you might require:
Smoothwall is a good choice for using an old computer as a firewall. Another choice might be Cobia.