There used to be some fairly fancy and expensive tail coat jackets out there from Great Britain and California , but to be honest, these shops may not be in business any longer. The stuff on Amazon and eBay may be cheaply made, and as you wrote, in smaller sizes. Given the time frame and the effect of the pandemic on the busines supply chain, and recent transportation issues as well (especially the US Postal Service), you might want to consider modifying a jacket of your own instead.
There's a "jacket modification thread" somewhere in this Anatomical section. I think it was mostly me who was writing on it and I focus on late 19thcentury military attire, so it may not be useful. Tail coats are more of a 1st half of the 19th century thing. You might want to look at the website ushist.com for ideas... They focus on historically accurate attire for re-enactment purposes, but they have civilian as well as military clothing examples. I don't recommend buying from them unless you have a fat wallet, though.
http://www.ushist.com/
