Another way to get a lot of ferrite cores for super cheap is to harvest them from cables that included them like VGA and DVI cables. you can cut them out and peel off the overmolding. Most are still split core. The material is often not very good at lower frequencies like HF, but you can use more cores or loop your cable through multiple times to increase the effectiveness of the choke.
> Mix 31 excellent for 1-10 MHz common mode suppression, then about same as 43 up to 250 MHz, NOT recommended for multi-ratio impedance transformers (baluns/ununs) due to material characteristics and power handling capability – ok for ham radio 1:1 feed line choke applications. Curie temperature >130 C
I know; i hoped for a site that sells all the types of rings, binocular, clip-on - which are great for choking VHF vertical dipoles with the bottom leg being spokes that are obviously too short for the frequency - i digress.