There are several DIY and natural cat deterrents you could try if you want to avoid buying a repeller.
Cat Repellent Plants
Cats have a very sensitive sense of smell and luckily are put off by smells that humans actually find attractive. So you may find a solution that puts of cats but actually enhances your garden for you.
Lavender, Rue, Curry and a recently developed plant the Coleus Canina or "Scaredy cat" plant all offer a natural way to deter cats. Both lavender and rue smell nice to humans, curry plants well as you can guess smell of curry which you may or may not want wafting through your garden. The Scaredy cat plant has lovely blue flowers in the summer and doesn't smell to us humans unless it is touched. To cats though it smells repugnant and is sure to keep them away from that part of your garden.
Adding physical barriers to the cats path is another way to keep them out. You may want to add spikes and chicken wire to your fence to keep the neighbourhood moggies out. This can be effective but can involve a lot of work and with no guarantee that your cunning feline friends won't find a way in that you didn't think of.