The official Ortlieb site wants $5 in shipping for a $3.50 buckle. I just can't.
REI sells some Ortlieb bike products. Don't recall seeing parts but you could call them. Also maybe tent and trails down by city hall.
I tried a buckle from Paragon (big sporting goods store, like an independent REI) but it was too tall. I didn't even think of that as a dimension as I was focusing on width and clasp shape. Considering what I put my panniers through maybe I should just get the OEM part.
You might try stopping by REI down on Lafayette in Soho. Back in the day, they used to have a wall of buckles, slides, webbing, etc. for rebuilding backpacks that was second to none. I try Paragon first to keep it local too, but they have never had a terrific selection of those sorts of things. REI might not any longer either, but it would be worth the try.
There's also an EMS on Broadway in Soho. Might be worth a try.
Don't get me started on the socioeconomic indicators the locations of these stores are for the Burning Man, Adventure Hippie crowd.
Got some cash
Bought some wheels
Took it out
'Cross the fields
Hit a wall
But we're alright
Try here: Side Release Buckles, Hardware for Sewing
You could also check with Gary Smith at BaileyWorks. He's around here somewhere.