The Omni is using Food Not Bombs as their 501(c)3 "fiscal sponsor". All the funds from their current IndieGogo campaign are being channeled through FNB, for example. FNB is providing this service for free. Most organizations do charge something, because they take on additional paperwork effort, and some legal liability on your behalf.
Another option we've discussed in the past for Sudo is
School Factory, a non-profit whose sole purpose is to help other hackerspaces off the ground. They are more expensive - if I remember correctly, they charge 10% of all donations (*not* including membership fees!) - but in exchange you get a lot of handholding in terms of learning how to do all the paperwork etc, and even classes on leadership, community building etc.
When we talked to School Factory for Counter Culture Labs, they strongly advised against doing a voting-membership style organization though, because of the additional legal complications compared to having a simple board-run organization.
For CCL, we decided to do the 1023 (501(c)3 application form) ourselves. It takes a bit of effort, but it's not too bad once you know what is expected. Kinda like writing Bylaws, really. We managed to dig up 1023 forms from a number of different organization, including response letters from the IRS pointing out mistakes in their applications and asking for more info.
Patrik