I purchased (with my own hard-earned cash 😀 ) a Layen Bluetooth transmitter for my iPod Classic some years ago and it works well. Layen still sell a similar model: this one for example http://www.layen-uk.com/Black-iPod-Bluetooth-Transmitter_p_64.html has the thirty pin connector that plugs into the iPod's dock connector.
There are other makes of transmitter. Here in the UK I have found makes by ONEU and IDC among others. Do an online search for "Bluetooth 30-pin transmitter" to find out what is available locally.
Alternatively, Layen and many other manufacturers make transmitters that plug into the headphone socket, but personally I prefer the thirty pin outlet type because it will provide the proper line out level (volume). This is important if you have an older iPod Classic purchased in the EU (European Union) as those have a lower maximum volume from the headphone outlet which cannot be over-ridden. So taking the Bluetooth from the headphone outlet of that type of EU iPod will result in low volume from your speakers.