I suggest opening a non-technical support case with NetApp to request the 9.8 vsim OVA. If permitted, they can supply it. https://kb.netapp.com/Support/General_Support/How_to_open_a_non-technical_case
Otherwise, reach out to your local partner manager (if you are with a reseller) or account team (if you're a customer) and your local solution engineer may be able to provide a link for you.
If all else fails, the vsim environment is upgradeable and downgradeable, such that you should be able to deploy the ONTAP 9.9.1 vsim OVA and then download the standard ONTAP 9.8 image and use it to downgrade the vsim environment to 9.8.
