The cheapest way would be to probably get a bunch of T12 2x4 troffers and take the ballasts out. 3 troffers will net you 6 ballasts. If you're lucky you can sometimes get them for free on sites like FB Marketplace or Craigslist.
There's a eBay seller
here with 4 Advance R-2S40-TP ballast for sale for $7 each.