It depends on what kind of 'bush' you start out with and what are the characteristics you like. Do you like small leaves or are larger leaves ok with you? Do you want a tropical topiary or something hardy in your zone?

You can start with something as small as a twig cuttings if you have the time and patience or go out to a local garden center and pick out something that grows well in your area or in your home if you have the space and good lighting.

But if you are like me and want/like immediate gratification I'd go with the larger bush and train it.

Also don't discount buying one. A locak garden center has a couple of 2-3 ball topiaries and they're only 30 bucks. I know 30 may be a lot but it's way cheaper than the 100+ prices I've seen for some.

Look around know and see who is having clearance sales and trying to move plant material.

HTH.