Where can I buy Tavist D and why has it disappeared from the stores?

Tavist-D is a combination of clemastine (antihistamine) and phenylpropanolamine (decongestant). Preparations containing phenylpropanolamine were withdrawn from the market some years ago because of the increased risk of hemorrhagic stroke.
see http://www.fda.gov/Drugs/DrugSafety/InformationbyDrugClass/ucm150738.htm

There are many antihistamine/decongestant products on the market that have the same effectiveness as Tavist-D. Your local pharmacist will be able to help you out.