Yoga is all about peace and serenity through poses that stretch your body, but you definitely won't feel serene if you are struggling to get into positions. A yoga block is a prop that's often used to help people into positions that they might not be flexible enough to reach yet. Once you get used to using a block you might need it for certain poses, so finding yourself without one can be a problem. Luckily you can use your imagination and find substitutes that will keep you wearing yoga pants and doing downward dogs as often as you desire.
Instead of using a block to help you achieve a pose, consider using a piece of furniture in your home or yoga studio. Yoga blocks help you to gain stability at times when you can't quite reach a pose, so in certain situations you can substitute a chair, bench, ottoman or small shelf instead. It's important to be flexible mentally as well as physically when practicing yoga, so learning to grab something nearby to help you obtain a position is a great exercise.
A brick or a wooden block that's the approximate size of a yoga block is a good substitute. Look for bricks or blocks at hardware or home improvement stores. You want the size to be around 9 inches long by 3 inches tall by 5 inches wide. These items are heavier than a typical yoga block, but can support your weight easily. Take care when moving a brick or heavy block around the room, or when you're lifting it over your head for a position.
Use a blanket, pillow or foam pad instead of a yoga block if you happen to have one handy. Your blanket can be folded or rolled up to approximate the size of a yoga block. Pillows or foam pads can be stacked or used alone. The important idea is that the soft object you're using helps to fill the empty space between your body and the floor. You can deepen your practice by using a real or substitute yoga block when needed, but be careful not to become overly dependent on any props for too long. Finding ways to move past using them can help you become better at the practice of yoga.
There are numerous things around your home that can replicate a yoga block -- simply be creative and look around. Try using a shoebox filled with a few heavy items, a box of facial tissues or a stack of books as a yoga block. Don't forget about the cabinet full of Tupperware either. Remember to be careful when using substitute yoga blocks, as they might not be as steady and weight-bearing as the real thing. Don't be afraid to improvise to improve your practice.