Are you talking about a re-recording mixer or an audio post service?
A lot depends on your distribution method. If your film is destined only for Youtube, Vimeo or an equivalent, there are no standards to attain and anyone with a few hundred to a few thousand dollars worth of equipment can call themselves a Sound Designer/Re-recording Mixer. So, it is possible to find a student or recent graduate to do pro-bono work, to build their experience. If you're looking to get into a festival (cinema playback) or you're looking at broadcast (TV) then standards are far higher, equipment requirements are far higher and a great deal more experience and skill is required. All of these factors result in considerable cost and mean that anyone offering pro-bono work for these distribution methods is either fooling themselves and/or trying to fool you!
It's worthwhile putting audio post into perspective, if you're looking at cinema playback (say for a festival). The audio post is going to take a minimum of 2 months and could take double or triple that long depending on genre, budget and difficulty/complexity of the feature. World class audio post is going to cost in excess of $1m and may cost as much as $10m or so. At the other extreme, a very low budget film, it's possible that as little as $50k - $100k (depending on genre) could get you pretty good quality sound. As you go below $50k though, your chances of getting decent or even usable sound diminishes considerably. Under certain very exceptional circumstances, you might even be able to get decent quality audio post for as little as $10k - $30k but there are
a lot of conditions attached to this price range, not the least of which is being exrtremely lucky!
Even for re-recording, a mediocre professional mix room in the US is going to cost about $500 a day, a Dolby mix room is going to cost a minimum of about $2.5k a day and a top class mix room can cost $7.5k+ a day.
With this information, what do you think your chances are of getting usable audio post and/or re-recording for $0?
G