Recommendation letters and testimonials are two different things. Ask for a recommendation letter if possible; it strengthens your future job applications. Ask your immediate superior if it's possible for him/her to write a recommendation letter for you. If he/she is a busy individual, prepare a draft recommendation letter before discussing with him/her so that when he/she gives you the excuse, "I am too busy", you can suggest your draft recommendation letter to him/her. This should work well in initiating the discussion of what should be written or included in the recommendation letter.

However, if you cannot secure a recommendation letter, ask for a testimonial. Most companies have a standardised template for the testimonial and as a bare minimum, the testimonial should state your scope of work, your employment period, and your starting and ending positions as well as the circumstances in which you quit (resigned on own accord, fired, retrenched, etc).

Hope this helps.