I'd like to improve my English for working purposes.

I have to improve my English for working purposes. That's why I would like to find some people who would like to communicate with me about various topics.

I'd like to improve my English for working purpose REASONS.

I have to improve my English for working purpose REASONS. That's why I'd like to find out some people who wants to dialogUE (CONVERSE) with me about various topics.

I'd like to improve my English for working purposes.

I have to improve my English for working purposes. That's why I'd like to find out some people who wants to dialog with me about various topics.

[Very good.  You find out something that was hidden or new news.  You find some people.]

I'd like to improve my English for working purpose.

I have to improve my English for working purpose. That's why I'd like to find out some people who wants to dialog with me about various topics.

I'd like to improve my English for working purposes. 

I have to improve my English for working purposes. That's why I'd like to find people who want( or, who would like) to talk with me about various topics.

dialogue*

Hi Roberto! :)

Only a few, easily fixable mistakes.The third sentence can depend on how you want to come across, "who would like" is generally more polite.