When you first start making soap don't worry about getting a fancy soap cutter.
A good sharp knife will do until you're comfortable with the soap making process and want to start perfecting your presentation.
Once you're ready, there are many types of cutters to choose from.
A popular and inexpensive choice for beginners is to purchase a mitre box and use a pastry scraper or cheese cutter as the cutting blade.
Be sure that the mitre box you wish to purchase is wide enough to accommodate your block of soap. Also be aware that you will probably have to widen the 90° cutting guide in order to fit a crinkle cut blade.

If you plan on making quite a bit of soap, you may want to purchase or make something a little more industrial.
My soap cutter was handmade by my Dad using the adjustable-wire bar cutter pattern in Catherine Failor's book, "Transparent Soapmaking". I love it and will never let it go. I might just take it to my grave if possible!
If your not lucky enough to have someone like my Dad around, you can purchase soap loaf cutters on-line from soap making suppliers like Willow Way who manufacture all sorts of industrial soap making equipment.