Shirts?

[kiltedsawyer]

I typically only wear solid color shirts, button down or polo, with my kilt as it is a tartan, Maclaren modern. Can't imagine putting stripes or other designs with plaids. Wal-Mart has some shirts, khaki, dark blue, etc that have two pockets with flaps, short or long sleeve for $10.00 And....they are 100% cotton which I also like as I try to stay away from synthetic fabrics. Just my 2 cents worth.
Respectfully,
David

[Zardoz]

Polo style or short sleeved broadcloth casuals in solid colors are great for summer if you want more than a t-shirt. And some lighter weight hose like W.Brewin sells,
http://stores.channeladvisor.com/wbsocks/Kilt%20Hose/
I personally like to match the hose and shirt, so when getting a new kilt or hose combo, I'll take the socks to the mall and match them up..

[12stones]

*** on the plain polos for tartan kilts. I prefer just a t-shirt, but since you're looking at business casual...that's the way for me.

[Tattoo Bradley]

Welcome from Southeastern part of New Mexico.

Like the other guys have already well stated, anything you'd wear with jeans and slacks can transfer over to the kilt.

[hospitaller]

Another alternative (very affordable) is to get Canadian Army surplus shirts.