With determination and good hunting skills you won't have to pay a dime for paving slabs. Sign up online with exchange and recycling sites, and make connections in your neighborhood or with friends to search for free flagstone. If you're happy to use broken flagstone, approach local builders or even quarries for discarded stones or rubble.
Check your town, county or online for free recycling agencies and put out a request for flagstone. These programs are often free to use, but you will need to pick up the flagstone yourself. Alternatively, review local trading and bartering groups that are based on members exchanging goods and services in lieu of cash. Such programs may use a system of credits or barter dollars classed by the IRS as taxable income.
Contact friends and ask them to pass on your request for free flagstone to people they may know. Ask neighborhood stores if you can put a wanted card on its bulletin board. Check demolition and new builds in your area, and politely ask builders for complete or broken flagstones that they're willing to give away. You'll save them having to shift the stone themselves, and it's neighborly to buy them a few beers as thanks.
Federal government agencies offer you the chance to haul several ton of free stone back home per year for free or a small annual permit fee, depending on the agency. You must do the work to remove the stone from its natural location, and haul it to your home. Contact the ranger district office of the U.S. Forest Service near you, or request a landscape rock permit from the Bureau of Land Management office in your state.