Tried the local auto wrecker? If they have a Mark 7 (or similar) it's reservoir is almost guaranteed to be there, and for cheap.
Are you positive the leak is from the reservoir? I thought I had one but upon further investigation, it was.. a radiator hose? No. It was from the radiator fins near the top of the radiator on that side... Corrosion finally ate through and pinholes were evident. A greenish discolored spot crumbled under my touch.
A new aluminum radiator was one of the best things I've given my old car.
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So, how have you tried to patch it?
No glue will stick, but since it's a somewhat thick plastic, you might try a soldering iron. Melt the seam or crack together, or plug a hole.
The technique is not too difficult. Practice on scrap first. An unregulated soldering iron is way too hot, so move quickly and try to avoid getting things so hot there's smoke. Smoke means the plastic is deteriorating, and the repair could be weakened when it solidifies. (Inexpensive plastic-welding outfits are available.. check HarborFreighttools.com )
You might use a piece of similar plastic as a "welding rod" to fill a crack or hole. I dunno exactly what type of plastic the reservoir's made of, but I'd guess it's some common, temperature resistant stuff.. probably HDPE (High-density polyethylene... milk carton). In my experience, any similarly soft, milky-white translucent (like a milk carton) scrap plastic is close enough..