The problem with pips players is that they seem to think about pips out rubbers the same way they think about pips in rubbers.
For pips in rubbers, yes you want to buy the rubber (sponge+ top sheet) as whole because if you try to glue a pips in top sheet to a sponge yourself, it would most likely be not uniform and thus in violation of rules .
For pips out rubbers there is no such problem. You can buy any Chinese top sheet & sponge separately. and glue them yourselves & it is highly unlikely to violate uniformity rules.
But the problem with Japanese & European pips out rubbers is that they always come as a rubber. And it is next to impossible to separate them . This is by design. They want to buy the whole rubber & spend more money. This is one more reason not to buy European & Japanese (pips out rubbers).
Of course personally I would not waste my money on super expensive Euro & Japanese pips in rubbers which are almost like anti spin (in the hands of a technique challenged player like me or in case of most amateurs but they won't admit it but that is a different issue altogether).
You should be able to but any sponge of any thickness or hardness and stick this to your pips out top sheet with no problems.
So stop limiting yourselves to pips out rubbers that you have to but sponge & top sheet assembles