You are correct.The natural pronunciation is /ə/ -- the reduced vowel or schwa. In fact, the schwa is used in all unaccented syllables, making it the most common phoneme in the English language. Some people believe that the indefinite article has a strong form /ei/ (personally I prefer /ʌ/), however, it is usually better to substitute a different word in such situations, for example "one," "a single," or "some."

Privet Ekaterina, Yes, normally the indefinite article would sound "ə". Some people use "eɪ" to give emphasis, for example, "I asked for a cup of coffee, not four of them," putting emphasis on the fact that it is only one. Similarly, the definite article, "the", is normally pronounced "ðə", but for emphasis, (eg. a superlative) it sounds "ði". That said, many native speakers will use one or the other, and nobody, except grammar nazis, will get too upset, so you will be understood whichever form you use. Regards, Simon.