Notaries often have a list of available shells and can set you up quickly. That's what we did, so talk to your notary. Another way is to use a specialized service like [1].

The relevant keywords for research are "Vorratsgesellschaft" or "Mantelgesellschaft". The former are empty companies with no business activity, the latter are older companies with business history but inactive now. Mantelgesellschaften are nice because they bring a credit history along making stuff like leasing easier but cause a little bit more effort at time of purchase.

[1] https://vorratsgesellschaften.dnotv.de/

We got ours from https://www.foris.com/en/shelf-companies/ and can recommend them.