The best ballast in terms of Fulham ballasts are Fulham Workhorse 5 ballasts.
If you want the correct ballast for your 90w SOX low pressure sodium lamp, here is one for sale:
https://www.ebay.com/itm/144635969047If you want to go the cheap magnetic ballast route like I did, I used a 70w M98 pulse start metal halide ballast with the original ignitor swapped out with an imported Philips SX76 2 wire parallel ignitor.