If you are certain the blower is out of balance (and it sounds like it is), you can go buy a new blower wheel. I am assuming that's the type of blower you have (squirrel cage, not a fan blade). Many times the blower wheel will throw a balance weight. It's usually more cost effective to simply replace the blower wheel. I have yet to find anyone who will balance them, not that it should be difficult with the right equipment.
The trick is getting the old blower wheel off. Many times you need a wheel puller which is a tool much like a gear puller. Once off, take it to an HVAC supply store (Johnstone, McCalls) and they should be able to match it.