When you searching for L.F. (=Low Frequency) generator you can find many choices.
For example: https://www.aliexpress.com/item/4000059352250.html?spm=a2g0o.productlist.main.3.515eEuO0EuO0Ws&algo_pvid=11a2198a-9cad-4fb9-9783-186f480df27f&algo_exp_id=11a2198a-9cad-4fb9-9783-186f480df27f-1&pdp_npi=4%40dis!THB!1176.41!576.35!!!33.74!!%402101effb17026883653417498e70b5!12000032443749804!sea!TH!712154667!&curPageLogUid=y1qQb1aDEqHX
But you'll need an extra cable, from bnc to rca: https://www.aliexpress.com/item/1005005996052778.html?spm=a2g0o.productlist.main.3.a9935edaigRKzI&algo_pvid=2f769e2c-9217-49dc-981e-24dd3f827bcf&algo_exp_id=2f769e2c-9217-49dc-981e-24dd3f827bcf-1&pdp_npi=4%40dis!THB!145.37!50.29!!!29.57!!%402103246c17026885643396680e0a72!12000035228125231!sea!TH!712154667!&curPageLogUid=KSIXXNXBTFv9
But I have to warn you about your speakers, because high frequency tones above the audible ranges can put a heavy strain on the speakers, so it is easy to blown up your speakers.