Well, seeds can be found aroun town but they're rather rare. You can also find them in tombs when you have WA. If you don't feel like searching for them, you can always type in testingcheatsenabled true and then buydebug. Under buy mode, there will now be a special category where you can simply plant any plant you wish in your backyard, also all spawners and tomb objects can be found there. This way, you can easily get your ten life fruits!
Quite strange that you would need life fruit for an invention... Maybe it's that rumored frankensim! How awesome would that be!