Get the best you can afford FROM REPUTABLE BONAI SELLERS. You can wind up spending some serious money on crap--especially if you're buying on places like E-bay.
Find a local club, you will have access to extremely good material for less than 1/3 what it might cost retail. Some club members might even GIVE you material to work on just to clear their benches (And because they're also good people). Over the years, I've been given stuff that would sell for hundreds. I've also passed it on to others.
And finally, don't believe people who say good plant material is more expensive in the east. That's not true. It's were you look for it and knowing the sources. Here in the east, expecting to get wonderful old Japanese black pine, aged shimpaku junipers, etc. is unrealistic. However, the oldest planting of Japanese maple, azaleas, boxwood and some other great bonsai species are East of the Mississipi. The East coast is home to the oldest and most sought after boxwood species for bonsai. You can find 100-year-old azaleas, as well as boxwood that are well over 200 years old. Many of those plants are growing in neglected areas. Those plants can sometimes be collected for free--less the time to get permission and elbow grease...