This sounds to me you haven't searched enough or have no clue where to look. No offense! There are a bunch of open-source wallet and library projects which you can study and fork.

Question is: what is your goal? What do you want to make different and presumably better than existing open-source wallets? Do you understand blockchain and coin protocols? Do you understand security models? Do you understand e.g. potential privacy issues of SPV wallets? There are certainly more questions to ask, but I'll stop here now, deliberately.