I just went through this -
I ended up ordering from Battery Cables USA and got 1 AWG cables cut to length with terminals and heat shrink for less than $60 total. They are very flexible and well made and exceeded expectations. They also shipped same day! I highly recommend them.
https://www.batterycablesusa.com
See the attached link from their website that lists all of the tech info like strand count, diameter, etc.
Be aware of the following info for some material options:
-PVC - least flexible, very resistant to oils and fuel (like off the shelf stuff)
-EPDM rubber (welding cable) - very flexible, abrasion/nick resistant, not fuel or oil resistant (as in ethanol containing fuel like in california)
-Silcone rubber - (not offered by battery cables USA) - I had ordered some Taylor wires from Summit that were silicone. VERY flexible and fuel/oil resistant. I was all set to install and then was reading that silicone abrasion resistance is terrible. So under the hood with engine movement and strapping in the wire, passing the wire past other metal parts, long term this may not be a good choice. Once it gets nicked, you could end up with a short situation eventually
-Extreme Polymer outer jacket (PTFE lined Engineered Thermoplastic) - very flexible, abrasion/nick resistant, oil and fuel resistant (I went with this choice)
Provide them terminal to terminal length, they will pre-crimp for you with heat shrink.