Unfortunately the answer is no, nothing can be done. Apps that are paid for via Google Play can only be installed and have the purchase validated but the Play Store. Even if you get the app from a third party, it will not be able to authenticate that you paid for it via the Play Store. If an app is removed from the Play Store, it will continue to be authenticated on your device as long as it is installed but if the application is removed for any reason it will no longer be available from the Play Store and the purchase information is lost.
Is it somehow possible to download these apps? Most of them I
purchased, but I can't find them even in "Library". I contacted their
respective developers, but got no answer at all...
The app is not really the issue, although a variety of different methods are used, the most common one is that app is the same for the paid or free version, but when you pay for it an authentication token is issues for the app on your device. When you remove the app the token is removed is as well, if the app has been removed from the Play Store, a new token will not be issued.
In most cases the developer can do nothing anyway, a sympathetic dev MIGHT give you an alternative to use the paid version, but I do not believe they have a way to validate that you purchased it from the Play Store anyway.