Ayahusca, ayuahuasca, and iowaska are not different substances. They are common typos or phonetic spellings of ayahuasca, the correct name for a traditional Amazonian brew used in certain ceremonies.
Most people using these spellings are searching for ayahuasca ceremonies. In the Netherlands, ayahuasca is illegal because it contains DMT, so any ceremonies are not officially permitted.
If you want a legal route in the Netherlands, alternatives include truffle ceremonies and sessions sometimes called psilohuasca or psiloflora. For a brief overview of ayahuasca and these spellings, see this explanation.
Hope this helps.