Fakemyip.info/proxylist contains an extensive list of transparent, anonymous, elite and other types of proxy servers. The list can be sorted by a number of factors, including proxy type, port, speed, country of origin and check date.
"Elite" proxy servers are likely to be the best fit for the purpose of being both anonymous and transparent. They are as secure as an anonymous proxy servers, in that they conceal the IP address of origin for the connection. While they are not truly transparent, they actively attempt to prevent the recipient from recognizing the connection as having originated from a proxy, thus making the proxy's IP address appear to be the original.