For around 50 bucks you can get a 15 gallon keg if you look hard enough...maybe cheaper. If you are a little handy with tools and have a 4inch angle grinder ($39 buys a good Skil brand at Ace hardware and others) cut the top out of it. Bobby sells all the weldless fittings you need for a drain valve AND a reasonably priced step-bit.
If 15 gallons is too big, mark it and cut the keg off thru the side lower, maybe at the 12 gallon size, bolt 2 stainless handles on and you have a boil kettle to last a lifetime.
I jumped from extract to AG all at once, bought 2 kegs for $90 total, ordered the bulkhead fittings from Bobby and added a sight glass tube and valves to both plus a used 10 gallon igloo cooler for $15. I planned to use one keg for a HLT, one for a boil pot and the cooler for a MLT. It all worked out for around $210 altogether fully set up, but I used my old 8.5 gallon alunimum brew pot as it was easier to handle, which is why I suggested maybe cutting a keg down some. I hate to cut a keg down but may do that myself.