Cucumis metuliferus is more commonly referred to as kiwano, horned cucumber, and horned melon. Its names suit its appearance; a melon-like fruit with large spikes all over. It's a vine native to Africa that bears spiny edible fruits. It is in the cucumber (Cucurbitaceae) family and doesn't look too far off from a cucumber. This is an annual plant that is great for growing in the garden, especially against a fence to let it climb. The plant can't survive cold temperatures so it is best grown as a summer crop. If planting outside, the seeds should be sown after the last frost or in spring. These don't make good house plants because being an annual, they demand a lot more sunlight than a window can provide. While the plant itself isn't very aesthetically pleasing, the fruits are. They usually have red, yellow, and green streaks.
Where can you buy horned melons? I would recommend high-end grocery stores, or specialty grocers that carry a wide variety of fruits. I am unaware of anywhere that sells seeds or plants so your best bet is to buy a fruit and plant the seeds.
This picture is a good example of the horned melon's climbing potential. You should allow the plant to climb up a trellis or fence rather than spread out on the ground. This will protect your fruits, save space and make harvesting easier.