The least expensive option after the built in flash is an ordinary flash in optical slave mode.
It won’t give you TTL, but it would not be difficult to make test shots to set the power…cheaper often means trading time for money.
The off camera flash in optical slave mode will fire when the on camera flash fires on the rear curtain.
If the exposure is long enough and you don’t want the on camera flash to fire, you can just fire the off camera flash manually…again without TTL. This would allow using pretty much any xenon flash ever made.