Load a transaction with bitcoinj
BitCoinJ is a Simplified Payment Verification (SPV) client library

This means that it only downloads the headers for transactions, and only those that are relevant to the private keys contained in its wallet only. This greatly reduces the blockchain download (very quick to get started), but at the cost of not seeing all the other transactions (reduced overall network security).

So, in short, you can't download an arbitrary transaction through BitCoinJ.

However, to answer your requirement have you considered just using the Blockchain.info API?