I feel your pain with needing long shirts--my husband is 6'3" so this is a constant issue for us. We've had great success at thrift and consignment stores. While most don't have a huge selection of men's wear, we're able to find good items by checking in frequently.
For new retail, I'll echo the prior posts on Costco and Kohl's--great sources. We've also found that Gap Outlet has amazing sales for menfolk. Pants and button-down dress shirts for under $10!!! If you have a Gap or Gap Outlet near you, check out their sales rack. Their shirts are fairly long, wish they were longer, but they're better than most places---they have a slim cut, which is nice and cuts down on bagginess.
The best long shirt my husband has is J Crew--we found it on the sidewalk in a box of free stuff (major score), it's the longest dress shirt we've ever seen! Perhaps you could find J Crew used at a thrift store? I'm definitely keeping my eyes out at Goodwill...