Congratulations for having the discipline to set priorities and manage your finances. That's smart, not "tool-iish"
$500 for two bikes in good condition is doable, but not easy depending on what type of bike and the riding you want to do. General purpose bikes come in different flavors - geared and non-geared. Single-speed (no gears) will be less expensive (generally) and work just fine if you want to cruise around park paths and local streets. Geared bikes help make hills and faster riding easier, but the extra components and maintenance can increase the expense.
Local garage sales and/or Craigslist is probably your best option for finding bicycles in your price range, but the caveat is finding ones that are in good condition and don't need service/replacement parts. If you need tires, tubes, a more comfortable seat, bearing adjustment, wheels trued, etc. and aren't mechanically inclined the cost of the bike immediately goes up. The more mechanically inclined you are, the more likely it is you'll recognize needed repair/parts before you purchase. Bearings that bind slightly, or have side-to-side play, cracked rubber sidewalls (tires) and other indicators are signs that some additional money may need to be invested. Either negotiate that into your purchase price, or move on to another bike.
If you aren't mechanically inclined or have little experience with bicycles, some LBS - not all - will be willing to perform a basic inspection for you before purchase. They obviously want your business once you become a bike owner and also will want to service the bike if it needs repair. The key is finding a reputable shop that understands your budget objectives, so you are advised about repairs that are critical and those that are "nice to do". Again, the less knowledge you have about bicycles, the more you will be relying on the shop for their experience. If they give you good advice, they are earning their money. Bike shops that offer pre-purchase inspections often sell bikes they take in trade. More likely than not, any used bikes you find in an LBS will be tuned up and ready to ride.
eBay is an option, but unless the bikes are near you, you'll lose the option to inspect them before purchase. Shipping a bike can easily cost $50-$100+, which eats into your $250/bike objective. Some assembly/adjustment is needed for bikes that are shipped, so again mechanical aptitude comes into play. The more knowledge you have of bikes, the more you will be able to inspect (zoom in) attached pictures for signs of service/parts that are needed.
Helmets are a good idea, so plan accordingly $$ wise. Depending on the riding you do, water bottles/cages and other small accessories are important or even necessary. Here's hoping you find two great bikes and are out ridding soon!!
