It is going to cost money to get formal credentials even if you find a place that will accept your portfolio as evidence of your competence (why would they do this for free?). Why not instead consider building a graphics career online as a freelancer while you keep your day job?
There are a number of sites where you can put up a portfolio of your work and bid on projects. My suggestion would be to start small, and work up from there: bid on small projects (say, company logos) and build a reputation on these sites. If you're good and have an impressive portfolio, you should be able to bid competitively for these jobs. The following are good and reliable sites for freelancers:
Creative Shake
Elance
Guru
Contracted Work
The first three have free membership options, but if you are serious about getting the best projects you need a paid subscription to bid on them. The fourth only offers paid membership options. Elance is particularly good at providing community resources for its members that help them land projects.
I would also suggest building an online portfolio if you have any website design skills (even if you don't, I can suggest website design packages for Windows or Mac OS that are easy to use and provide professional results). That way you can give people the link to your online portfolio - a good thing to have on a business card even if you're applying for jobs locally.