I bought one from Ebay, was about the size of a button, and just wired it to the dashboard (green) indicator bulb.
My bus only has one dashboard indicator indicator bulb - that may have been a PO bodge, but if easy enough to fit 2 if you've left and right bulbs - or fit
direct to the flasher unit...
.....was a while ago, probable one similar to this....
https://www.ebay.co.uk/itm/Piezo-Buzzer-Sounder-3-12V-DC-85Db-BUY-TWO-GET-A-THIRD-FREE/122062736986?epid=27002949646&hash=item1c6b818e5a:g:3IIAAOSw42JZI-~~