Welding shops are a good bet. They usually trade out a full one for an empty one. If you want to keep your tank, you might check a fire extinguisher company. They typically can fill your bottle so that you can keep it.
If you were planning to keep your bottle, post a pic of it. You shouldn't need an adapter. Do you already have a regulator for it?
there are a few liquor stores that will fill it. Home brew mart has a list of them. I go to liquor locker on Morena Blvd, its about $16 for a 5lb fill. Air-gas will do an exchange as well. The advantage there is that you will be getting a full 5lbs instead of 4lb or so from a typical fill place. disadvantage is that they charge whatever they want (I've had them try to charge me anywhere from $18-$29 for a 5lb tank).
wow...good call on Westair. they're near my work and paying 18 instead of 16 to get one free means the 6th fill is only 10 bucks! worth it to me....thanks!
I also use Liquor Locker, I think it's like $3-4 per pound but it's close to my house. Beer King on Clairemont Mesa (behind the Guitar Center & Arby's) used to do it but I don't even know if they're there anymore.
Just thought I'd revisit this thread and post something current. I get my bottles filled at Keg n Bottle on Massachusetts Ave in Lemon Grove. $17 for 5# and $22 for 10#. Price has varied depending on who's working. They prefer frozen bottles and they do not use a weight scale; just a straight fill till it won't take anymore. Used to go to Beer King on Shawline but I don't think they're there anymore?