http://www.parachute-cord.com/
can use that stuff, if you want to use leather, then there is ( if you're in an area where leather is easily attainable and at a fair price ) is some pricey things.
For instance i'm soon going to be buying a 22 sq. ft. cowhide side, for $85, shipping included in price.
That'll probably get me about 1.5-2 whips ( likely 2 since i'll be doing 2 bolsters and 2 bellies instead of 4 bellies )
But you need other tools. For instance i'll be using an Australian strander to cut my strands. You can get this for about $24 ( all this
is in USD ), or if you are an elite member of tandy leather you can get one for $10. Also you need to make a beveler for your
strands, and a splitter will help, but if you find a place like i did, who's hide ( specific one i'm buying ) never goes above 4.5 oz,
and never goes below 4.0 oz, then a splitter is not 100% necessary. ( difference between 4.0 and 4.5 oz is only 1/128" )
Plus you'll need leather dressing, which isn't very pricey, i'm not trying to discourage you, i much enjoy making whips.
Just if you are on a very tight budget concerning this you might be better off using paracord from that link i put at the top.