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This article should help: https://www.gardeningknowhow.com/ornamental/flowers/lantana/how-to-propagate-lantana.htm

Lantana will not survive your winter outside. If your cellar stays above 32*F., the lantana will survive, but require light. Fluorescent bulbs of warm white and cool white will give the light spectrum needed to overwinter.

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It could be the time of year and/or lack of water. If your temperatures have dipped below 50*, the plants got cold. Lantana will throw leaves when dry to conserve water.

I would recommend doing a local search for your area to find any retailers or even mail-order catalogs. Your local garden center may also be able to help. You can also check out eBay. In my experience, eBay is actually a great place to find unusual plants/seeds. Just check the feedback on the seller before you buy.

This article should help: https://www.gardeningknowhow.com/ornamental/flowers/lantana/lantana-pruning.htm