I picked up a '58 250 last month, and after a couple longer XCs (FL->MI, MI->PA, MI->AR) I realize that the 4 hours until silence from the motor isn't quite long enough.
Doesn't matter if they are new or in good used condition. If new, who are the manufacturers to look for? A google search for Comanche Tip Tanks only yields for sale results for entire aircraft, not just the tanks.
T(h)anks,
Ed