Go to your local garage or mechanic's. Ask if they have any spare leftover paint from a car paint job.
Contact any paint and decorating firms in the area and ask if they have any leftover paint.
Enter online competitions. There are plenty of competitions for budding artists on the Internet. Some will require a submission of work and others will require the answer to a question. See graffinc.com for links to graffiti websites providing competitions.
Offer work in exchange for supplies. Contact local businesses to see if they want a sign or display for their store. Offer to do it for free if they pay for supplies and you can keep the leftover paint and materials.
Create a portfolio of work. The more you can show your talent, the more you will increase your chances of a likely investor in your talent.