Ditto on checking office stores. Mine is from Office Max.
Your best bet is to look through the Sunday ads for the places that are close to you. You can look them up on-line today and 'shop'. You definitely want to 'test-drive' them first. Weight is not a real issue for these chairs as long as you get a metal 'spider' on the bottom. The real difference is in the seat width. My chair that I have currently is almost 4 years old and I love it. I used it when I was over 500 lbs and still use it today after my surgery at 275 lbs less.
You will want to take into consideration the floor that you will be on when you are using your chair. When I first bought my chair, I bought one for hard floors because that is what I had. When we moved my new room had carpet and the 'different' surface ended up breaking my base - it was extruded/reinforced plastic. I loved my chair so much I bought a replacement base that is metal with deep carpet wheels.
Don't be afraid of getting one in a box. They are really easy to put together. Good luck.