When I converted to partial mash brews, I used a 3 gallon drink cooler and a large reusable grain bag. I was only mashing about 5 lbs of grains so that worked well for me, plus it had a spout. If you are going all grian soon you could get a five gallon ine and convert it into a mash tun.
Bill from Pa
On Deck: Irish Stout,
Primary: Pumpkin Ale
Completed: Red, Wit and Blue, Irish Stout, Red Ale, German Style Amber Lager, All Grain Brews: Irish Red Ale, American Stout, Honey Weizen