http://wiki.i2p-projekt.i2p/wiki/index.php?title=DNS_registration
Mostly, these services are hosted within an existing registry, since a registry already has a lot of domains (by nature), increasing the chances of a jump URL being given successfully. A selection of jump services (put a hostname like "i2p-projekt.i2p" at the end of one): Jump services URL Info http://stats.i2p/cgi-bin/jump.cgi?