I bought some balloons online. Will they blow them up if I also buy some balloons from them?

I also asked this...unfortunately they won't. The manager did recommend to buy a helium tank at Walmart (party section) to fill them up. We did., and turned out great!! ~Hope this helps~

I bought balloons from somewhere other than dollar tree and they would not fill them for me.

Yes. They might charge for helium though.