In my 1000 gal septic system the pipe that goes from the tank to the leach pit is out of round and is only about 1 inch exposed. I am trying to attach a coupling to it so I can attach a tee to create a baffel so the paper won't enter the leach pit. Is there any product that I can insert into the pipe leading to the pit with a collar that I can then attach my tee to?

First, no paper should even get that far. Paper usually dissolves in the sewer pit.

As far as the fittings for your pipe go: each pipe has numbers printed on it. Expose more of the buried pipe and record those numbers. They tell you what schedule and type of PVC you have. Buy fittings accordingly. Most of the time you can get them in Home Depot type stores.

You can also measure OD of the pipe with tape-measure to figure out size of your pipe.

If Home Depot doesn't carry couplings of your size than you can purchase Flex Seal coupling . This coupling can be used on all pipes regardless of what material they are made of.