I think that the shields from the WHFB Warriors of Chaos would be great, they aren't round shields like in 300, but they are built and shaped in such a way to make for a great Phalanx, in addition there are a lot of cool pieces in those sprues to make your marines look a little more gothic. Take a look at my Highborn Techmarine WIP thread, those legs came from the Warriors of Chaos + a little greenstuff.
The actual spartans were heavily armored warriors, and most actual Phalanx formations made use of tower-style shields, in addition, the WoC sprue "walking" legs are perfect due to the nature of phalanx style warfare, in which the soldiers' advance was normally a slow march to a battle-hymn or drumbeat. You may also note the notches in the WoC shields are nearly perfect for notching a spear through. A little GS work would square out the pointed bottoms of the shield for a beautiful tower shield.
You may also consider another range of spear, the DE spears and all hooked and nasty, more like glaives than anything else, the HE spears are more suited to thrusting attacks.
Feel free to disregard all of this in favor of style, just throwing some ideas out there.
Well if push comes to shove you could purchase some plasticard and a metal pipe with a slightly larger diameter than the sheild size you want and punch out your own sheilds. This would be the most work of the above mentioned methods but would allow you to get whatever size and look you want with work.
I would agree with the plasticard route for round shields, I spent some time bitz browsing and came up with squat for what you have in mind. Some plasticard circles + a little greenstuffing to round out the business side and you could have a pretty neat conversion.
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