You can find lots of debs (those that you have installed on your Ubuntu desktop, that is) in /var/cache/apt/archives/ . The advantage of that is that you don't have to go hunting very far. If you can't find the deb that you are looking for there, try searching http://packages.ubuntu.com
Once you have the packages that you need, go ahead and change the debs into rpms using Alien. The command looks like this:
> alien --to-rpm the-package.whatever.deb
With that done, stick the package someplace where you can download it to your Midinux device, then pop over into the X-Terminal and into the directory where the rpm you need to install is sitting. Install the rpm with:
> rpm --install the-package-whatever.rpm
And there you go!