I presume you mean for the Panther, on which this auxiliary armour was most commonly seen?
The Griffon set 35A001 does seems to be hard to find. BNA Modelworld in Australia have stock. I've used them many times and found them reliable.
ET Model also do a Panther A/D/G set including turret roof armour (which the Griffon set does not), EA35-061, and a Panther F set EA35-070. BNA, Der Sockelshop in Germany & lamaquette.be in Belgium have the 061 set I've used Der Sockelshop often too: just had a box from them today, in fact.
The Griffon set and both ET sets are on eBay from Australia (BNA again) and China. BNA shipping can be cheaper if you use their website, especially for multiple items. The Chinese eBay seller is Kingman Model, who I've also bought stuff from without problems.
Buying in Europe saves the risk of customs charges, although a single packet of etch would probably slip through. Der Sockelshop have a very good range and their prices and shipping aren't bad. Likewise Modellbau Koenig in Germany and Jadar Models in Warsaw. Browse any of their sites and you risk spending a fortune! Other stores are available ......
Try Google or eBay searches for "Panther armour 1/35".
Hauler do a set in 1/48, HLX48320 and ET do a set in 1/72, E72-010.