Can I find any reliable public archival nodes / Indexers?

Questions from TG:

Is there any reliable public archival node ?

and

I assume to build a proper indexer which relies on all events it would need an archival node ?

Usually archival nodes or indexers are offered as a paid service. You can also host your own. Additionally you can create an indexer without it being a FN. It is possible to set up a custom structure that suits your needs and it can hold only historical data relevant to your application.
You can definitely have both and for that here is a relevant page in the sui docs that might be helpful