Passenger side mirror leaking water and part is now discontinued, any ideas?

[180fan]

So like the title says, my 89 240sx has a water leak from the passenger side mirror and the spongy stuff is now discontinued by Nissan. I'm looking for other suggestions as to what I could use to make the seal that the spongy stuff isn't doing anymore. I was thinking about using some closed cell neoprene sponge, but I forget if it's water proof. Any other suggestions before I blow a small wad of cash for two small triangular pieces?

[Ni2s4s0aSnX]

silicon sealer.