You could have a look on Ebay or Amazon.
Maybe a fan is offering to make or sell some on a Doctor Who fan site.
There is also a chance that they are available in fancy dress shops.
Failing that, you can try making your own.
You cannot buy most of the originals. The few surviving pieces are in museums.
Colin Baker (the 6th Doctor Who) had his replica costume for conventions made by an independent costume maker. My other suggestion is do the Comiccon and other fan conventions.
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In the episode of the third series of Doctor Who called 'Gridlock', the Doctor states that Janis Joplin gave him his coat.
The coat is made from a dark olive green moleskin fabric. It was custom-made for Matt Smith in his role, but seems to draw on military style, from the storm vents and the double-breasted closure. Replicas of varying prices and quality can be found at Baron Boutique, CosplaySky, Magnoli Clothiers, and, soon enough, Abbyshot Clothiers.
It's Solomon's coat.
You can find some on eBay, as well as on other online sale sites. DON'T EXPECT the screwdriver to work as in the television series; Doctor Who, and much of the technology used there, is fictitious.
karamati coat (1994)