Plenty of P-channel MOSFETs to choose from.
The SPD50P03 is 30V, 50A, 7mOhm, TO252 and the IPP80P03 is 30V, 80A, 4.4mOhm, TO220.

The Recom switching regulator modules are very nice but if the current drawn by the low level circuit is low(ish) then a linear 7805 is just fine for going from 12V to 5V and a lot cheaper (about 1/5) than the Recom module. And IIRC the Recom module needs a minimum load for the regulation to work - which usually isn't a problem but something to look out for.

/Henrik.