我很长时间找了一个练习造句子的网站很久，但什么网站都找不到。／我找了一个网页对站来练习做造句子很久，但不会什么网页站都找了不到。

In your original sentence, "I've been looking for a website to practice my writing for a long time.", the "to" would be better translated as "来" since it's indicating a purpose/goal (i.e. to practice my writing). "To" can indeed be translated to "对" but usually it is used like "对我来说，中文很难。" (To me, Chinese is difficult.)
Or, it would be more natural to just use the whole phrase "practice my writing" as the adjective to describe 网站, in which case it explains the purpose of the word 网站. It's like saying "I have been looking for a website in which I can practice my writing."
And for the past bit "什么网站都找不到":
找 = find
找到 = found (past tense)
And if you add "不" right after the verb "找", it becomes:
找不到 = didn't find

在这里，我看到一些别的帖子，还有看起来很多人改了它们收到很多改正。

It would be more coherent to add "在这里" (on here), since you weren't talking about this site in your last sentence, it might confuse the reader as to what you're referring to! :)
And it would be more natural to say the posts "收到很多改正" (received a lot of corrections) instead of "很多人改了它们" (a lot of people have corrected them). Yours would make sense to a native speaker but it would sound a bit weird.