it's only logical to assume that some sulfuric acid produced (at the end) is looped back into this reaction.
This would be the easiest method, as the production of sulfuric acid is continual (not batch), so from when a suitable output is established and maintained, a portion of the final product is reintroduced in the absorption tower to keep the production going.
as for where the sulfuric acid was first obtained from when the production first begun: other sulfuric acid plants
as for where the sulfuric acid was first obtained from the first sulfuric acid plant: it is a naturally occuring compound

ON THAT NOTE: the absorption of sulfur trioxide into oleum is only an optional step, to avoid producing a sulfuric acid mist as the reaction of sulfur trioxide and water is higher exothermic. so this step is not essential to the production of sulfuric acid. If they wanted to get some sulfuric acid to start off with, I'm sure they can just react some SO
3 with water... It'll be very hot but.