If you want Super 4 track you will have to purchase it second hand.
Good quality Super 4 track commands surprisingly high prices with some pieces being in very short supply. Tri-ang Society membership has increased 13% in the last 12 months so that would suggest a resurgence in interest in Tri-ang material from the 1950's and 1960's.
See if you can pick up some R476 converter tracks which will connect up your old Super 4 to current Hornby track. Surprisingly many new locomotives will run on the old track but there are some older locomotives that will not run on the new as they have deep flanges on the wheels.
I do know that Hornby have their knockers for sticking with track that is considered to have toy track dimensions but there are many thousands out there who run collectable and secondhand locomotives and they (like myself) are no doubt grateful that Hornby take their needs into account.
Happy modelling
Gary