StarTrek.com , Roddenberry.com or Avonos do not offer it, nor have I seen that particular costume piece offered for sale before. You may be able to find a blue lab coat like it by doing a search through your favorite search engine.
Find a high quality picture of the lab coat wore by Dr. Crusher & make sure it's what you want before buying. Ebay, or other sites may have it for sale. Search for interviews with William Ware Theiss or Robert Blackman, TNG's costume designers & see if it is mentioned, or locate their webpage / email & ask if the lab jacket was custom made or a purchase from a store. Contact Gates McFadden by email & ask her, too.
Also you might check out Star Trek Costume sites w/ the search, Dr. Crusher's Lab Coat. Memory Alpha shows 2 versions of varying lengths & colors. Click on the hyperlinks for more information. TrekFan1984 Ends (talk) 01:09, May 20, 2017 (UTC)
A quick google search turned up this one on Hello Cosplay: http://www.hellocosplay.com/star-trek-the-next-generation-tng-beverly-crusher-coat-cosplay-costume.html-Cpthunt (talk) 18:44, May 21, 2017 (UTC)