Hi Bev,
I think Fred has some good points here. Don't count on all tubes, even from the same supplier, to be the same! Get a dial caliper, and check the sizes you need. If the part that fits inside the tube is .285 you know it isn't going to fit inside a tube that has an inside dimension of .275. You should have better luck buying tubes from the same supplier as the original, so you may want to do that to increase your chances. But you may also want to look into getting a set of metric reamers. This will assure you that your inside dimension is what you want. Harbor Freight is a place where youcan pick up both the caliper and the reamers, but I would suggest you also try Ebay.
You may want to start by seeing if you have a burr around the opening of your tube that is preventing what would otherwise be a good fit. Bill Baumbeck sells a neat little de-burring tool that I highly recommend. But you can remove a burr as easily as running the blade of a pocket knife around the inside opening of the tube.
But you really want to have a dial or digital caliper, and measure those tubes. Good Luck!
Scott.