I don't know how to post yet, since I won't really know for a couple of days whether it worked or not. It felt like a disaster almost every step along the way, but I am still hopeful that it worked out, and will be as amazing as I have dreamed. So I will post what I did and learned so far, and then once I know, I will post the results, and perhaps this post will be a useful lesson in either what to do or what not to do.
Challenge : Find the materials. Most "recipes" for concrete leaf castings were something like : 1 part Portland cement, 3 parts sand, and some part water and/or bonding/fortifying agent. But as I suspected, the brands and products mentioned were not available locally, so I did some investigating on the Home Depot web site, and went to the store with various product descriptions in my hand, hoping to finalize the decision. I came away with two bags I couldn't lift : 88 lb of Portland cement, and 80 lb of jointing sand. I had investigated various mixes, but all included some gravel in the mix. I wanted mine to be a fine mixture, so I chose to mix it myself, and also chose the finer jointing sand rather than construction sand.
The Home Depot store didn't carry the acrylic based bonding agent which I was seeking, but I persisted, and phoned the number on the web site, and asked them to track down the product, which was a Stone Mason "Acryli Bond" liquid. Sure enough, we found a store not too far away which carried it, and I was able to pick up a 4 L (1 gallon) bottle.
Challenge : Dust masks. They were advised for the mixing of the Portland cement, which is a fine powder. Good thing I bought a 3-pack, so my son & daughter had fun trying them out. Since I couldn't wear mine. I felt like I was being suffocated, it was too hot, and my glasses steamed up when I breathed. So after trying a few times to use my mask, I gave up on it, and just held my breath when scooping the cement, and tried to mix carefully. I pre-wet all the sand, so at least it was not dusty at all.
Here's my daughter on the porch of our shed, where I was setting up the Gunnera leaf on plastic, and mixing up my sand and cement in the wheelbarrow. The wheelbarrow was a good idea. I wouldn't have managed with the deeper bucket I had originally planned. And it washed up really well afterward.
[Aug 31 : See result, which was not as I hoped].