Are there any online dictionaries that supply IPA transcriptions for Mandarin?
You are unlikely to find such a resource. For languages that already have an unambiguous standard notation for pronunciation (which pinyin is), it's uncommon to find dictionaries that indicate pronunciation in IPA. Heck, it's even hard to find dictionaries with IPA for languages with incredibly complex orthography/pronunciation mappings!

Instead, as the comments on your question suggest, you should find a resource that provides pinyin to IPA mappings, and then you can mechanically derive IPA from pinyin if you so desire.