No junkyard around these parts so I made one from pieces I bought at Home Depot (think they come in 18x24 sheets for around $18...dont remember exactly). No welding needed. Got two pieces, a couple of heavy duty nuts, bolts, and washers (can use heavy duty wire) and bent the hell out of them to fit (had the help of a heavy duty vise). Then used the bolts to secure the bends.
If dont want to do that, just look in the phonebook for a cimpany/outfit that works with metal. Describe what you want, take them pictures, and they will make you one more likely than not.
I have seen the ones like luv2camp recomended at Fred Meyers and Home Depot in stainless steel. I dont know how big your fire box is but you can probably make one cheaper. Try going to a second hand store like goodwill or value village. See if they have a chicken roaster or small turkey roaster that will fit in your unit and drill it full-O-holes. A steel pot or cake pan might work also. Or an old ammo can with the paint burned off and filled full-O-holes might work. If you dont have a drill take it out back and shoot it with a 22 a bunch of times.
Possibly off topic, but slightly related: anyone ever thought about adding a vertical charcoal stack, like the one used in a Stumps smoker, to your firebox on the horizontal smoker. I was thinking about removing the door and building a new one with a huge piece of square tubing welded through it. Put a sealable door at the top and a large piece of grate at the bottom. Make it big enough to hold 20 pound of charcoal. Would have to build a support on the firebox or the smoker would flip, I guess.
I made on myself last week out of scrap expanded metal and tack welded it together. It was actually my very, very first welding job. But you can go to Home Depot buy a piece of expanded matal for about 10 bucks, grab some brass all-thread stainless washers and nuts and make one. Just take the expanded matal and bow it to fit into the firebox. Use the all-therad with washers and nuts to hold the bow. Simply remove to refill and clean the firebox using welding gloves purchased at Harbor Freight.
Here's what I welded together. I'm not welder but I find it very fun to do. It's a blast.