How old/big are your boys? If they’re still pretty young you will struggle to find historical-style tunics off the peg, in which case I’d get them plain cotton granddad shirts in the smallest size and a neutral colour (you can belt them in/roll up the sleeves if they’re too long) and call it done:
Burgschneider do good solid basics - see the Child’s Eriksson tunics and the adult Erik, Lodin, Tyr and Aegir. HOWEVER they’re in Germany - shipping is fast but you’ll still be cutting it very close.
At this point you might be better waiting until you’re on site and buying before time in, certainly for yourself. Chow’s Emporium and Ceolred Monger usually have solid generic tunics with slight Saxon/Viking detailing, which is what you’re after as Steinr.
Good suggestions from SevenSecrets above; also look for Velvet Glove at the event… or you could even order/reserve something to collect at Anvil: http://velvet-glove.co.uk/index.php?page=6
It’s probably too late for the ones “by Velvet Glove” since those are made to order (but you could ask), but the other ones are from stock… e.g. the two-colour style might be good, or maybe the Gadaric… and there are children’s ones.
Another relevant ebay search, this is for “tunic” items that the seller has listed in the “larp” category: