Maybe they've been to a function you missed. Let them know ahead of time you'll be looking, and ask them to keep their eyes and ears open. If the first two suggestions don't work, your job gets tougher. You might have to research online. Probably the worst way to choose a DJ is on price alone.
Some DJs are more expensive than others.
That's quite a difference, and it would be very tempting to choose the cheapest alternative. If that's all you can afford, than you have no choice. But, consider the law of supply and demand. There is a reason some DJs can charge more money than others. They are usually worth it.