Matodextrin powder should be available at your homebrew shop as well; MD will add body without much sweetness, whereas lactose will add both body and a faint sweetness.
However, are there problems with the beers as-is? I know it's a little hard to judge until they're carbed up, but try a sample now. Just because they finished low doesn't mean they don't already have all the body and sweetness they need. Lower levels of carbonation can also help beers at lower FG have a fuller mouthfeel as well.