Keeping it simple is very appropriate for Bonsai. For outdoor stuff I often use Redwood, particularly if it is going to be exposed to moisture. My reason is that it is a 120 mile round trip to get more exotic stuff. The end grain has to be as close to straight up and down as I can find if I don't want it to warp.
Since you're not using much wood for this, you could use one of the Oaks described above, or even order a few feet of something exotic online. You're only going to need 6-8 feet of 6 inch wide stock, which you will glue up to make the top.
The more I do, the less I accomplish.