Take appropriate photos or draw illustrations yourself if you can. If you aren't artistically inclined, enlist the help of a friend who might be willing to help out for a small fee or credit in the book. This is the easiest way to go about securing illustrations for your book.
Research artists and enlist those whose work you like. Propose a small fee, exposure through a credit in your book, or even a small percentage of the book's royalties for the right to use some of their illustrations in your book. Many beginning artists welcome the opportunity to get their work in print and will be willing to make a deal.
Consider a collaboration with an artist or photographer. If your book will need a number of illustrations, it may be worth an actual partnership with an artist that doesn't require you to invest any money up front. Based on a partnership, the artist would get paid from the sale of the book the same way you do.
Take advantage of public domain photos and drawings, creative commons license photos and drawings, or low-cost royalty-free images. There are a number of these illustrations on the Internet. Many websites offer a variety of illustrations free or at very low costs.