While ornamental sweet potato vines ( Ipomoea batatas ) are most commonly sold as established or slips during the spring, you can find them from specialized nurseries and major garden retailers. If you specifically want to start from tubers, some vendors offer dual-purpose varieties that provide both ornamental foliage and edible harvests. Top Online Retailers
For local pickup or quick shipping, these large-scale retailers carry various ornamental Ipomoea brands. Go to product viewer dialog for this item. Ipomoea Batatas 'Margarita' Size 6 9" By White Flower Farm
: Known for selling sweet potato slips, they carry varieties that are selected for both their ornamental vine performance and their tuber yields.
These nurseries specialize in ornamental varieties and typically ship rooted plants or slips that will quickly develop the tuberous root system you're looking for.