Ebay- cheap but take your chances with quality.
Every town has several suppliers of vinyl signs.
Designing a vehicle livery is a skilled job and an investment, not a cost, I spent over £500 on mine, and spending over £1000 is not unheard of among carpet cleaners.
In my opinion if you DIY cheap, then you look cheap and and you are detracting from your brand. A well designed Van will earn its costs back many times over. Have a look at bossdog .