I purchased a 20 quart Stainless Steel salad bowl as a cheese vat. They are easily found in any Restaurant Supply store either online or at B&M. The online stores even have a 30 Quart available. The 20 Quart is around $12US and just just barely is washable in a standard US sink. In fact when I rinse it I have to use another cup to pour water over the bottom of it
as it doesn't quite fit upside down into my cheap apartment sink. I don't have the 30 Quart but it runs over $30US and I would imagine be even more difficult to wash in a standard sink. Now, even 20 Quarts equate to 5 gallons, it is not possible to put 5 gallons of milk in this bowl as it would be absolutely to the brim and overflowing. 4 gallons is the absolute maximum that can fit in here, well sanely anyway. Here's a photo of mine in use, with a double boiler:
