You cannot make it from phosphoric acid easily, if at all. I'd recommend buying it. 

Edit: You could do 2(H2O) plus P4O10 which will lead to 4(HPO3), but that sounds like more fuss than it's worth.

Diphosphoric acid is synthesized by heating the orto-acid to 215 degrees C.

2H3PO4 = H4P2O7 + H2O

If you heat H3PO4 to 316 degrees C, metaphosphoric acid is synthesized. It exists in cyclic and polymeric forms and has the brute formula HPO3.

HPO3 is very strong dehydrating agent that could be used instead of acetic anhydride in RDX, HMX, TNT and other synthesis.

It could be possible to prepare HPO3n from normal 85% H3PO4 (1, 5 dolars per liter) by heating (could it be concentrated to 100% by heating?).

Then 100% H3PO4 (melting point 42,3 degrees C) will be heated slowly to 316 C (electric stove will work).

Anyway it is unclear how long it must be heated to the upper temperature. To get 95% + yield ot HPO3n.

what do you think about HPO3 and H3PO4? do they hava the same effetcts on Vitamin C measure??

I'm not familiar with what you are measuring, but I'd expect HPO3 to form a cyclic phosphate with ascorbic acid. Phosphoric acid would probably do nothing to ascorbic acid since it is not electrophilic and it is not oxidizing.

Thank you Brennan. Since i can not get HPO3 from H3PO4, do you know other product similar to HPO3. 

And what kind of reaction would have Acid ascorbic with HPO3? i guess both are reducers??!!!

Thank you