Helms 24 in Distress
There are a number of used boat gear shops around the country so it would help if you mentioned where you live or have the boat. It is pretty unlikely that you will find a used boom that actually works properly for a Helms 24 and even if you found one that was the right length you will probably end up having to purchase a gooseneck that fits the hardware on your mast.
Boom extrusions and end hardware are not all that expensive. You might look into just buying the parts new from Dwyer or Kenyon, both of whom have good websites.