Nigel, most the Home depots have great upright varieties like Italian cypress, and its either san jose cypress or san diego cypress, I can not recall the name, these are for $10 to $39 dollars depending on the size. They also carry elms and oaks in ten gallon pots with great straight trunks that could be chopped this time of year, and parsonis junipers and blue points which are straight trunked juniper varieties. These would all be from 3 foot to ten foot tall, but they are reasonable and readily available at the home depots here in orange and osceola as well as polk counties in central fl. The home depot on hwy 27 in polk county at the eagle ridge mall near the road to winter haven fl. has an amazing buy on several weeping figs, which I beleive are Ficus benjaminas, they are ten to twelve feet tall with trunk around six inches diameter and great strong radial nebari, you could make several air layer over time and chop the trunk to about a foot and have an amazing bonsai! these are a special sale for $20, I would have bought one but I already have 6 tropicals and no time to make the layers before I go home. And they would be to big to transport home in my RV. They are worth checking out though.
ed