Dear Barb, Chuck, Loren, and mid-60s Buick engine bay tweekers, . . . .
Hi guys does anybody have an over flow tank for 65 Skylar . Please and thank you.
Well that depends on how involved you want you install your coolant recovery tank. You see my Dad and I first installed a tank with an elastic "bungie cord" around 1980 or so and that lasted . . . oh, two decades or so!

In 2010, I bought a nice universal kit that I even painted carefully.
It was not very expensive and the installing was very straightforward. Unfortunately, these kits appear to have gone extinct. That is very unfortunate. That would be my first recommendation.
Alas the engine in my trusty wagon didn't last more than 2 years after that. In 2013 I went another route and bought a nice metal tank. To be exact I bought this model of Mr Gasket recovery tanks.
https://www.summitracing.com/parts/mrg-9133At the time it didn't seem like such a bad idea because it was about 1/2 the price you can find them now.
Ouch!!I'm not sure what to recommend. The Mr. Gasket tank does fit nicely on the edge of the radiator and it doesn't take up a lot of space.
It is late around here. I'll need to go back and look again. Right now I don't see a coolant recovery tank solution that will be easy to install and look reasonable in your engine bay at a moderate cost. I'll try again tomorrow.
Wishing all mid-60s Buick owners a pleasant night's sleep!

Edouard . . .