Question edited for clarity. the last section need some extra words and desperately needed punctuation as a huge run-on sentence.
Finding stuff is what I do best. That model number appears to be flawed. The best Search Queries are those that are the absolute shortest.
https://www.google.com/search?q=100145+4343
Who makes it? If it is 'Rainforest' rather than a descriptive term for a lot of water over your head, the company doesn't exist. Is it a beautiful knock off of Grohe, Delta or Moen made in China?
https://www.google.com/search?q=Rainforest+Shower+100145+4343
Generally, cartridges are available for at least 10 to 20 years.
There are lots of cartridges here, but not for a model number your stated.
https://www.google.com/search?q=100145-4343+rainforest+shower+replacement+cartridge
One solution might be to remove the cartridge, descale it, replace O-rings and reassemble it with a good smear of water pump grease which is waterproof.
If you can fill me in with any further details, click Comment under THIS Answer, not up above under your question.
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