You can use svxlink on a raspberry pi (if you can find one?) and a behringer UCA222 (for audio) to link two repeaters directly. SVXLink is really really good- but it is not easy to use.
The developers recently added svxreflector, so you could build a network. Again, its not easy to setup but its very very powerful. If you go this route let me know- i'm going to be building a linking system for my repeater and will run a reflector. I don't know who i would link with at this point tho.