I used the internet to search the world to track down unusual varieties and species. Its much easier these days. For the white beach plum, I had to track a rumor to find someone that had a couple of plants growing that produced the fruit. I got some seeds from them, but the plants from seed produced purple fruit, so had to plan t seeds of the cross pollinated first generation offspring to finally get a single plant. Alas, it is not growing well these past two years, so I am trying to take some cuttings and save seed. But I am not as young as I once was the the quest becomes harder to pursue.
Regarding tomatos, please check out rareseeds.com, (Baker Creek Heirloom Seeds) a family seed company that produces amazingly beautiful color catalogs, and easily sells at least 70 tomato varieties. Another good place to try is totallytomatoes. Rareseeds sells something called an Oticali berry, a member of the nightshade family, that I have grown for a few years now. It does not take up a lot of space. If you want to try something really bizarre and scary, try litchi tomato.
I've grown it off and on for years. That one initially took me more than a year to find a seed source, but that was pre-internet.