My advice is to start with either a Procumbens Juniper, or even better, a shimpaku. All the ones sold in the landscape trade are usually sold as ground covers. They usually have rather coarse foliage.
For bonsai, we're looking at trees as if they are under a microscope. We want nice, fine, compact foliage.
Procumbens does that. But the various forms of shimpaku are even better.
Where do you get them? You can find Procumbens at landscape nurseries. But shimpaku will mean you need to get some at a bonsai shop, usually. Once you have one, you can take cuttings off your tree and have as many as you want.