My old ducting is worn out and has several leaks in it and it is hard to get to. Is there a flexible type duct that I can pull threw the old duct work and make new connections at all the registers for a better air flow.

Just what do you have now? The old tin or fiberglass stuff should last forever. If some of the flex stuff is going, you might do better to fasten the new to the old and use the old to pull the same size new in. Anything that fits inside the old is going to be smaller and reduce capacity and flow.