Can somebody help me out there please

My car is a 1994 c220 diesel auto

I have a leak at the return pipe going into the fuel tank i replaced the small pipe with rubber flex and removed the drain plug to find the oring was shot i went to the stealerswhip and requested a replacement oring .
The storeman said the oring was no longer available and would need to buy a new sender unit to get the part i wanted this is nonsense can somebody help and give a rookie some help cheers