were can I find a after market radiator overflow tank. dealer want to much.
Junkyard
Google.
Same car parked in a dark alley at night.
You didn’t way what kind of vehicle it is, so that could matter. Since things like that are often not stocked at typical AP stores, I often start with a quick search of craigslist. People are often parting out cars there. That’s very hit and miss though. Then I’d go to the salvage yard option. Then I’d end up at one of the online parts things such as rockauto since many of them source more kinds of things than the brick and mortar AP stores. If it gets hard to find, I’d be willing to try a patch using some plastic epoxy, depending on how bad it is.
Also look on Car-parts.com for junkyard inventory.
Sometimes you have to go to a “pull your part” yards.
Depending on what make/model it is there might be an aftermarket one available. Auto zone for example sells Dorman brand tanks and picking a fairly common car(2000 Ford Taurus) they normally stock them for that vehicle for $60. They probably don’t stock ones for every vehicle but any not kept in the store are most likely available within a day or two from the distribution center. Year/Make/Model would help us.