Answer: As always, plant size descriptions represent a range and there will be variations depending on season, growing conditions, and overall climate. Some of the confusion and somewhat wide variations you found on this plant may be due to the fact that the original size descriptions were based on the plants being grown in containers rather than in the ground. In a container, one would expect the plant to be a bit smaller than its full potential when planted in the ground with an unrestricted root run. Also, in areas where the plant is winter killed to the ground, it will be naturally smaller than in areas where it is top hardy (nominally USDZ zone 7b). Finally, remember that some catalog descriptions indicate shipping size, not necessarily mature size. You may find the description from the National Arboretum interesting, this is part of a series introduced by the Arboretum.
Keep in mind too that a well established, fully grown, mature plant raised in the ground under ideal conditions in a perfect climate will likely be more to the high end of the estimate and may even surpass it.
For sources, I would suggest you begin with the nurseries listed in the zip code locator at the Monrovia web site. Many nurseries may offer only a reduced plant selection or limited availability at the end of the season, so if your local nurseries do not have it in stock currently it is possible they will be stocking it or could obtain it for you next spring if you ask. They may also be able to tell you what to expect with this plant in your local growing conditions and in the particular location you have in mind.
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Good luck with your crepe myrtle!
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