Additionally in the game Inquisitor characters can be armed with Melta pistols, there very expensive but they are there, meaning that on at least some level they are available.
Additionally in the 40k RPG Dark Heresy, the Inferno pistol (under the Melta weapons section in the inquisitors handbook) is available to own for the cost of 7500 throne gelts (imperial currency) on the open market of any sutabily high enough tech world. They are described as Very Rare (which is the 2nd highest rarity, scarce being the highest) and have a insane armour penitration of 12 (civilian power armour *non astarties grade* has an armour value of 12-14) so its clearly a melta weapon. Given thats open market, i think its probably not too inconceveable that the ultramarines could lay there hands on a few, or make there own given there nature.
the defence rests, there ARE melta pistols out there, i have pictures & rules from books considered cannon to GW's 40k universe. Thus they exist.
