As you will see from this USDA Plant Profile, the Cordia boissieri (Mexican olive) is native only to Texas. You can follow the plant link to our webpage on this plant to determine what its growing conditions are, and try to figure out from that if it will be able to survive in the climate, soil and rainfall of Marion County FL. At the Lady Bird Johnson Wildflower Center, home of Mr. Smarty Plants, we recommend only plants native to North America but also to the area in which they are being grown, in this case, Mason Co. FL.
Frankly, we don't hold out much hope that you can find this somewhat rare tree, endemic to Texas, but we suggest you go to our National Suppliers Directory, enter your town and state or just your zipcode in the "Enter Search Location" box and you will get a list of native plant nurseries, seed companies and consultants in your general area. Each has contact information so you can find out if the tree is available.
Also at the bottom of our webpage on this tree, click on the link to Google on that plant, with the possibility that a nursery who does shipping might have that plant available. We did find one article on Cordia boissieri (Mexican olive) which said the plant was quite rare and may be on the edge of extinction. From Dave's Garden, also read this negative comment from Odessa, Florida:
"It begins to grow and even flower, but as soon as the wind blows a little, the entire head or at least various branches break off. I lose a year's growth with a puff of wind. It is more soft and breakable than hard and brittle. Prehaps it is too wet for it in Florida. Anyway, its headed for the compost pile."
We did notice, on this search, a few nurseries advertising Cordia boissieri (Mexican olive), so you might check for possibilities.