Pinyin to Chinese characters
I suggest that you shouldn't do this.
Chinese characters cannot be faithfully constructed backwards from a tone+syllable combination -- the mapping only goes one way (and even then, sometimes characters have multiple pronunciations).
For example, as you know, 馬 is generally pronounced ma3.
However, ma3 could also reference the characters 碼 (number), or 獁 (mammoth). Without context, you do not know to which I am referring if I simply write ma3.
As you amass a vocabulary, you will start to notice patterns that will give you an intuition for which character is meant by the English pinyin, with or without tone markings.
The following two Pinyin sentences are equally comprehensible to someone with a beginning Mandarin level, as the context far dominates tone markings:

ni shi bu shi wo de pengyou
ni3 shi4 bu2 shi4 wo3 de5 peng2you5

The reader would assume:

你是不是我的朋友

Eventually, soon, you will not find it useful to use tone markings when typing Pinyin. Because it is a hassle to type, and any IME worth its salt will give you the contextually correct characters. This is probably the reason such converters don't exist.
I suggest you focus your efforts instead on learning more vocabulary and reading as much as possible. The false answers from your Pinyin IME suggestion will become obviously incorrect in your eyes.