You can use pine or alot of other types of wood that come in 5/8. Are you going to have a CP overlay or anything to cover the grain? If so, then I would say get whatever you can find in 5/8.
Also, go to Home Depot and look through their pre-cut wood. They should have an area in the back of the lumber section where they have "scrap" wood. You can find a lot of pieces that other people had cut there, left overs etc. It may take a couple of trips but you may find a piece of MDF that has been pre-cut that fits your needs. And those pieces are VERY cheap.
I would suggest you ask someone in the lumber dept to keep an eye out for you for MDF if you cant find anything else. If you ask politely, you may find someone to grab what you need and set it aside for you.