Look up monoammonium phosphate fertilizer. I'm unsure how "organic" it is, but it's certainly water-soluble unlike most other forms of phosphorus. It's the salt form of orthophosphate, which is a form of P that plants can uptake immediately.

If you want a slightly slower acting, more organic input then manure is a great source of P. Compost is also good.

That aside, are you sure you have a P deficiency? P deficiencies are rare whilst P lockout is common. P has a narrow pH range where it's available, outside of that it starts become immobilized in the soil. Have you been overwatering or adding excess calcium? Anything that could upset the pH?