Red Wigglers. But since just about anything can be called a Red Wiggler, you want Eisenia foetida.

Can you pull them from your garden? Probably not. Worms that are good for composting generally don't live in garden soil.

I think I ended up with two different kinds of worms. In my rubbermaid bin, I think I have mainly European nightcrawlers. In my wooden stackable bin, I think it's more of the traditional red worms.

You can pull them from your compost heap. I did. If buying I would want to make sure they were Eisenia foetida. I might have a mix. I would prefer not to. Some types might be more inclined to crawl. Some experienced worm wranglers prefer a mix.

Is it okay to start out with a mix taken from a compost pile?

From a compost pile yes. Then later if you decide you want species specific worms you can put the original worms back in the compost. Be sure to tell us if it ended up both types were identical. I would prefer to have Eisenia foetida. I do not know what I have. Maybe half Eisenia foetida and half something I like less. Mine are free from my outdoor compost. At one time we let a few red wigglers from a library workshop into the outdoor compost. You might want to practice with compost worms and then move up to the $ worms when you feel confident of their sucess. Just put the first worms back into the compost to not contaminate your new worms.