I've got about 18000 spark arrows after selling and rebuying stuff (deadric cuirass, I think) from on of the two armourers in the plaza of the Foreign Quarter. What is nice is that his stock keeps refilling on those.
Small spoiler:
If you buy his stock of arrows, wait 24h, then sell the arrows to him and rebuy them, he'll restock an amount equal to those he had restocked and those you sold him, making you able to multiply his stock of arrows easily.