These belts use a lot of paracord. I bought a 1000 foot spool of black military-grade 550 parachute cord from my favorite military-surplus supplier, The Sportsman's Guide
. There are lots of places to buy it, but I have used this site for other purchases multiple times, and have always been treated well.
Amazon is a good source for colored paracord, but does cost more than the "standard" military colors of black, green, and beige.
Belt buckles can be as expensive as the moon, or you can do what I do and buy a belt from Goodwill. I spend an average of $3.00 on a belt, cut the buckle free, and get rid of the belt. Both the single-tongue pass-through type and the big flat buckles with the prong underneath work. If you use the flat one with the prong, though, you will need it to have a long prong. My husband has a lot of trouble keeping his fastened because the prong is too short.
Other things needed that bear less need for in-depth explanations:
heat for melting/sealing paracord ends
small needle-nosed pliers for pulling cord through tight spots
measuring device (ruler, tape measure, etc)
cutters (scissors, sharp diagonal cutters)
I also recommend either masking tape or rubber bands for making long lengths more manageable.