I'd skip the Harland boxwood. It's not all that winter hardy in your area in a container. That complicates your care for it and will likely lead to frustration. It means you will have to find an inside (ish) area to keep it in the winter. Bringing it into the house for the winter will result in a weakened tree, spider mites and dead foliage. If you really want a boxwood, get a locally sourced temperate zone species. The boxwood sold at landscape nurseries or even Home Depot (buxus sempervirens, or Buxus Microphylla) are rock solid bonsai candidates that can take winters outside easily.
Boxwood can also be sourced from old landscaping as they transplant pretty easily. Local hedges and the like can turn up nice examples. The tall boxwood below --a buxus mycrophylla--Japanese boxwood--was dug out of a hedge 20 years ago and has been developed into bonsai.
Trident maples are extremely forgiving and easy to work on. They develop much, much more quickly than boxwood of any species.