How does somebody get to enter support for Army Special Forces (SF)?

Contact an S-1 NCO at one of the SF Groups. Even better, find somebody of a similar MOS (or at least in support in general) in SF and ask them about their own experience. They will probably have some tips for you, and could possibly provide a warm introduction. There are many members of the SF Groups on RallyPoint, so using the advanced search on desktop would be a decent way to start. Either way, good on you for pursuing such opportunities!

What the sir said. Also there are usually SF recruiters that leave pamphlets at the PX with their contact information. That'd be a good place to start. Most of the SF recruiters I talked to were more than happy to point you in the right direction, provided you showed initiative and maturity. Bottom line is that these guys deal with a ton of people asking questions and hate having their time wasted by people who don't know whether they're actually interested or not.

Another place to check is either shadowspear.com or Sofrep.com. Both of those are fantastic SoF communities with tons of enablers and operators who are more than willing to help. And since you are looking for a support role, talking to people with the job you want will help. Just understand that they expect you to do the research yourself, so don't show up unprepared.

If you are not near a main Post and don't have access to an SF recruiter, as a last effort get in contact with a local National Guard armory, and get the number for SF Accessions. These are usually the guys that are in charge of recruiting for guard SF units, and most of the time they know the units in your area and will at the very least get you in contact with a local active duty SF recruiter who will be able to answer your questions. Just be up front about your intentions and don't beat around the bush. They don't have to help you, but if you are respectful and show them you are motivated enough they most likely will help you out. Its usually a SFC who is in charge of getting the new Guard SF hopefuls ready for selection.

Bottom line is network, talk to your chain of command, and do research.