Barracuda Networks has some web-filtering appliances, but, again, this is not an inexpensive solution.
For commercial (paid) appliances, you not only have to buy the hardware itself, but you almost always also need to purchase an annual support contract, so that you can receive updates to the firmware and detection signatures. These costs can add up quickly.
As Doctor Olds said, an inexpensive used PC with some PCI expansion cards (to provide at least 2 Ethernet ports) plus an open-source software package will be the most cost-effective solution.
I don't know if this list (»en.wikipedia.org/wiki/List_of_ro···ibutions) includes everything that is out there, but it lists quite a few options. Not all of these will have the filtering capabilities which you desire, but you should be able to find something here which will work for you.