Nepal Trekking Guide Advice

Hi,

I am going to Nepal for the first time in a few months with a couple friends and am planning on hiking the Annapurna base camp trek. I was just wondering if anyone had advice about guides for these treks. Is it easy to find a guide once we are already in Nepal or should we pre book one? And if your answer is pre book, where can I do this? I understand that there are trekking companies who will sort all this stuff out for you but they charge a lot of money and we are trying to be a little cheap.

Thanks!