The ACE Hardware stores in our area have carried sheet styrene for a little over a year. It's in a cardboard display rack. Rather pricey. For thin sheet, I use "For Sale/Rent" signs. Very inexpensive. For thick parts I use gift cards or old charge cards. Gift cards are everywhere from Mickey D's to grocery stores and they're free. I especially like the ones that hang on a rack. Some of them have plastic that is more than twice the size of the card itself.
Rods are harder to find. I collect drink stirrer sticks. I guess you could by them in a store. Around here they mark utilities with little flags attached to 1/8" styrene rods. Lawn services use these also. They can be collected in the evening or you can call diggers hot line and they will deliver them to your residence for free. Scale wise they are rather large, about 3".