Where can I find the Proxy Library for the ENC28J60?
The proxy layer is abstracted from the hardware by many levels, so we really don't have any examples to share. The proxy protocol is implemented on a much higher layer than the traditional Ethernet MAC controllers. That is why no examples exist. You will need to combine the examples provided with higher level code to implement the proxy layer.
The proxy layer is very high-level software and the examples for the ENC just cover the driver layer. You will need to find a proxy example that calls lower layers of the Ethernet protocol and put that together with the examples for the ENC28J60.