Translation:These objects are quite difficult to obtain.
It's an adjective in this case. Or rather, "difficult to obtain" is the adjective phrase. ("Assez" is an adverb modifying that phrase, and you can leave it out to see the rest of the sentence more clearly.) "Ces objets sont difficiles a obtenir" has the same structure as "Ces objets sont rouges", except the adjective is a phrase. Anyway, it still has to agree with "objets."
Please see this http://french.about.com/od/expressions/a/impersonal.htm
It's like a passive voice usage. When you use it in another way, you don't have to add à
We want to say something. Nous voulons dire quelque choses
We have something to say. Nous avons quelques choses à dire