When you say the rear lift bar, are you asking about the whole set up, or just the lift bar that goes from the hitch plate to the tiller? Do you have the main rear lift bar between the under carriage lift link and the rear hitch plate? Here are some ballpark numbers for you, for purchasing the required items needed for your attachments:
Front lift bar: $35.00 - $55.00
Under carriage rear lift set up: $125.00 - $175.00
Tiller lift bar: $35.00 - $60.00
Helper Spring: $35.00 - $65.00
Rear wheel weights: $125.00 - $175.00
If you have the short lift bar that is attached to your snow blower, it can be used as the tiller lift bar as they are identical pieces. When you say mule drive, I'm assuming you mean the one required for driving the tiller? If not, you can expect to spend another $125.00 - $200.00 for it, and then another $100.00 for the drive belts.
As for a flat trailer, no John Deere doesn't make a flat trailer. A long time ago, they made a Model 20 and 21 flat trailer, but they are so rare to find, and if you did find one, it will cost you more to buy it then it would for you to just make your own.
Welcome to the John Deere world my friend. Where, in order to get green, you have to spend some green! The tractors and attachments are easily found, but the lifting and drive assemblies are very scarce and expensive. Good luck to you on your search.