For doing open segment work, I use eBay all the time. There are several companies that make 'thin lumber' and if you search eBay for that you'll see that there are lots of choices in everything from 1/8" to 3/8" in a number of different species (fex exotics, though). They are all sanded for thickness and when I apply a micrometer to them, they are usually accurate to within a few hundredths.
The boards are usually about 3" wide and 18"- 24" so that they can fit in a USPS Priority Mail box. I keep stacks of each species near my saws so that I can just grab one and cut it into the widths I'll need. This approach lets me cut segments for a project very quickly.