You only need to tack on the な when it precedes a noun (this goes for all な-adj's). The な is used as a means to connect the adj to the noun. If there is no noun following the adj, there's no need for a な. 丈夫な財布 > a sturdy wallet. 財布は丈夫 > the wallet is sturdy.
In casual, less polite speech you could use この財布は丈夫だ, I believe either だ or です is obligatory. But it would be better if you consulted a grammar guide or something like that, as stuff in my head gets mixed up very easily... Sorry.