Sign shops, sign shops, sign shops. I can't stress this enough. While you can find sintra for cheap online the shipping costs are astronomical and drive the prices up really high. Also keep in mind that sintra is just a brand of expanded pvc. The same material goes by other names as well such as foamex, celtec (the brand I use), etc... Don't be afraid to call around locally!
I live in Georgia and managed to find a place called sign-works unlimited which doubled as a graphic designer location. They had a warehouse full of the stuff. For $80 I got two sheets in total, each 96" by 48", of 3mm ($30) and 6mm ($50). That's not the kind of price you can find online.
If you are struggling to find local shops you will sometimes find that if you call the major plastics companies that are nearby they can often point you in the right direction if they are feeling helpful. That's how I found my supplier.