To get things that "count as" big choppas, go down to your local and trade some bits. Mainly look for ogre bits, as their weapons are bigger than anything the orks currently have. When you have a weapon that is the same size as your ork, there is no question that it is a big choppa.
Alternatively, you can make your own from the bits you must have in your bits box if you are building anything but AoBR models. Go get you box and do a quick rummage through; here are a few ideas.
Idea 1, use some sprue as a handle and put two axe heads one over the other on it, (like flags on a flag pole).
Idea 2, take a couple of your spare choppas and cut off the blades, glue the blades together on yet a third choppa.
Idea 3, take the left over parts from vehicle and use them as such, the klaw from the grabbin klaw makes a good PK, the wrecking ball makes a definitly big choppa stand in, puting a large piece of armor on a stick would count too.
Customizing, and kit bashing are just one of the joys of the ork army. Feel free to orkify anything, there is no way to build orks wrong, just let your opponent know at the start of the game as to what is what. For example; that the model waving a wrecking ball over his head "counts as" having a big choppa, etc.
And since we are bringing up the subject of customizing and kit bashing, have your heard of this?