The basic answer to your question is ... No.
The reason nobody makes a "universal" grid is that the market for it would be extremely limited. Most of the softbox manufacturers make or supply their own grids.
I suspect your best bet is to find an existing grid that's close to what you need and then modify it.
There is a fabric grid in the Westcott Lighting catalog and it's $189.90 (40 degree fabric grid for a 24 by 32 inch softbox). A similar grid for a 16 by 22 inch box is $139.90 and one for a 38 by 46 inch box is $309.90.(Wow,that kills any notion I had of buying grids!) Of course, those are manufacturer's list prices and you might be able to do considerably better.
You can always keep an eye on the used market (eBay, and the on-line camera places. Tallyn's Camera has a used area that sometimes has bits and pieces of studio gear. I think B&H, Columbus Camera, and Samys used equipment lists might also be good places to look...
Another alternative is, as mentioned previously, to make your own. They are only strips of material sewn together. You could probably do something with light cardboard and paste, or maybe hire a seamstress... be creative!
Gosh, $309 bucks... that's amazing...