https://www.1000bulbs.com/category/t5-reflectors-8-lamp/
Any of these work for you? Can get 2, 4 or 8 bulb t5 fixture
If can get the info off of the ballast you can replace it with another HO ballast, may need new bulb holders wich are cheap, depends if has been rewired or not, there is normally 2 holes so you can cut the wire and plug in a new ballast.
here is there ballast page, just look at wires needed and the voltage and dont forget what kind of start you want. All the info you need is on the ballast in the unit you have now, or can change to a differant start if you wish. The ballast are easy to replace, 10 minutes can take old 1 out and install new 1. Just cut the wires of at the bulb holder and insert wire from new ballast in the hole, if not a 2nd hole cant get new bulb holders at lowers for couple dollars.
https://www.1000bulbs.com/search/?q=HO th5 ballast
They have a hydro section if need any grow supplys too.