Has anyone encountered a decent soul trap bow early in the game? If so, where?

Yes but it would be pointless telling you where because they are randomly generated, you just have to be lucky enough for one to turn up early on in your game.

You could just enchant your bow to have a soul trap enchantment.

This. You only need a 1 second Soul Trap enchant for it to capture a soul on death. Especially useful for assassins.

Drunken Huntsman

There is the...

'Steel Battleaxe of Fiery Souls'
Soul Trap = 5 seconds
Fire Damage + 10

Can be found at...

'Ironbind Barrow'
Nordic Ruin NW of Windhelm.

Ideal to disenchant and reuse on a bow.

Thanks for that info am, makingin an assassin next !!