I don't know if this can help but Mouser has the FETs you looking for 511-RFP15P05 and 511-RFP15N05L listed in the No.608 catalog. Also Allied Electronics has them too, RFP15P05 = 263-2066 and RFP15N05L = 263-2063.
I have built the SDS Labs headphone amp using Int'l Rectifier IRF620 and IRF9620 with great success. IRF610 and 9610 could be a better pick but the '20 works like heaven in my amp. Mosfets available at digi-key.com.
I drive a pair of low impedance Grados with it and the amp is dead quiet on idle, have great dynamics and precison.
And yes, you should really put some resistance (I put 221 ohms) in series with the gate of each mosfet or the amp may oscillate.