The iPhone 3G and 3GS both had a 3.5mm headphone jack. There are lots of places to get cables with a stereo mini-jack on the end.
These phones also had a 30-pin dock connector. I believe you needed to use that connector for loading music from a computer, and for charging, so presumably you have something to do that on hand. As far as finding any sort of clock radio or speaker unit with a 30-pin dock connector, your best bet for that would be to check thrift stores (Goodwill and Salvation Army) on a regular basis in hopes that a used one might one day turn up.
(Pretty much all of the iPhone audio accessories on the market today are designed for Lightning or for USB-C.)