Where can I acquire a replacement plastic bulb (float) for the fuel gage sending unit?

Good Luck finding one. Measure the Length and between the groove where it attaches to the the bottom of the sending unit. I had to replace mine about a two years ago. Can't remember where I got it, but I do know the new one "groove" measured a couple MM larger but it worked. I think I found it on Ebay. Maybe try some of the Euro Cars? 

Any vintage Fiat or Alfa dealer. The actual sender won't interchange but the mfgr is the same. Some floats are sold plastic, some are hollow. They interchange but your gauge calibration may move. Careful bending of the hardened steel wire arm can readjust your gauge.

use a piece of a cork  sanding block 

Simon