You can buy on online shops the nichrome wire, however it tends to be expensive and they sell just a few amount of it, meaning that if you make a mistake you will need to buy more again and wait more time for it to arrive at your home. The best solution is to found on local shops, typically on ones that sells parts for electric machines. the same shops usually also sells the insulation.
See here a picture of the Casainho extruder using nichrome wire and insulation, both bought on local shop.
There are a few online shops that sells nichrome wire: