Hmmmmm...........A couple thought for you
Tortoise shell is a tough one.
Here are some of the things I know for certain:
1. The only legal tortoise shell is that which was made into a product prior to the CITES enactment, the Marine Mammal Act and I believe there is one more law that addresses sea turtles as well. These products have had to be made prior to 1973.
2. If you take one of these products, let's say a shoehorn, and cut it up to make scales for your knife, you are in violation of all of the above federal and international laws. The law that makes it even stronger is the Lacey Act. If you re-fabricate existing items, you turn it back into raw materail. The raw material is illegal even to have in your possession, not to mention it violates all the other laws governing endangered species.
3. Finsihed products (referenced above) which are legal to own still cannot be shipped internationally "for commercial purposes" and there is even some question as to whether these items can be moved from State to State within the United States.
4. Even artists who do restoration work to existing tortoise shell pieces are in violation of all the laws mentioned above because they are converting some pieces back to raw material to use to repair other items
I would suggest that you not concern yourself with using actual tortoise shell on your knife. The penalties are stiff if someone wants to make an issue. Believe me, there are a few Fish and Wildlife people who want to make an issue. For the most part Fish and Wildlife officials are very knowledgeable, informed and dedicated to the laws they enforce both in fact and in principle. Tortoise shell issues are serious for those folks, as they should be. They are good people, and they do their jobs well. Don't make a problem for yourself.
Now, as to potential look-a-likes. There are some possibilities:
1. You can purchase celluloid scales from some suppliers. I think Culpepper carries the celluloid scales. But, you run the risk of ruining the knife from the gasses that come off the celluloid as it ages. Also, celluloid is flammable and a wee-bit dicey to work with if you are not careful. So, celluloid is a tough choice.
2. There are several plastics out there that try to imitate tortoise shell, but as you said, they do a lousy job. Most of these plastics are new. To be honest, there is little incentive to duplicate the tortoise shell look in plastic these days because it is just not popular. So what is being done, just is not done very well.
3. You can purchase antique caseine scales on occasion that have great pattern and color. Just remember that caseine, while looking like plastic, is actually a natural material made from milk proteins. It will expand and contract, as well as shrink over time. There are things you can do to make it easier to use the caseine, but remember that it is a delicate natural material and needs to be treated like one.
4. One of the best options for getting material for scales, is antique pieces made from bakelite and other more inert plastic (not celluloid.) If you shop ebay, you can find some outstanding material. There are many things that you can do to make it look even more like original tortoise shell. While I won't go into them all, the most notable way to spice up that nice piece of bakelite is to gold leaf the liner before putting the plastic scale down. This was actually a common practice with tortoise shell in the past which was used by better makers. Tortoise shell is semi-transparent. To get the light to reflect back out, and to cover the metal of the liner, gold leaf was used. This gave the tortoise shell that extra rich look that you expect to see. It can have the same effect with antique bakelite. Gold leafing is simple and yes, please use the real gold. Real gold not only adds to the value of the knife, but it adds to the authenticity and the overall beauty of the bakelite. You can create a beautiful effect using well-chosen antique bakelite and gold leaf. (just a note, keep the piece of bakelite you cut in the event there is any confusion about the material on your knife. If you ever need to prove it is not tortoise shell, it is the easiest and non-destructive way to do so.)
I hope this helps. I've tried to include as much as I know about the tortoise shell. It has taken me years to accumulate this knowledge. I have acted as a confidential informant on several occasions for the U.S Division of Fish and Wildlife. I was part of the team that convicted and jailed the ONLY foreign national in history. She was smuggling elephant ivory out of Africa. So, I'm not making this stuff up. I've lived it and lived with it for many years.
If any of my experience and knowledge has, or will help, it is my priveledge to share it.
Be good.............
Tim