If you use a backer when making intarsia, Baltic birch plywood is one of the most popular materials.
It’s easy to cut, takes adhesive well and stays flat.
Right now, Rockler have a deal on 24″ x 30″ sheets, 1/2″ thick. I know a lot of people use quarter-inch, but this 4-sheet pack is half price at the moment. If you don’t want it for backers, there are always plenty of other uses round your wood shop (I know plenty of woodworkers use this stuff for home-made jigs).
Click here for full plywood offer details. I think the offer is only on until the end of the week, but it may be longer.