If i sold my thieves guild armor, is there anyway of finishing the theives guild quest for the achievement?

I am trying to complete the achievement of bringing the thieves guild back to its glory!

Nope, you are essentially screwed. This is why you should never sell unique items.

Pretty sure u don't need it. plus u get nightingale armor pretty soon.
or try buying it back

Actually, you need to become guildmaster of the thieves guild in order to fully restore it to its former glory. However, you get the guildmaster armor by turning in you starting guild armor so if you lose it, you can't become guildmaster and thus not get the achievement.

The way posted will destroy your achievements but will let you finish the quests if you are on PC. There is one dealing exactly with that problem.
http://www.gamesas.com/index.php?/topic/1303999-unofficial-bug-fixing-pc-only/