Cantillon is an inspiration for many, myself included. I am not a barrel expert, though I have 2 small barrels (7.5 gallons and 5 gallons).
French oak (or European Oak) has a finer grain structure due to a more moderate climate. It is said to let less O2 diffuse into the beer. American Oak can have a stronger flavor, but we don't want oak flavor for lambics.
My barrels were new, unused, cognac barrels. These had to prepared with a sequence of washes with barokleen and water rinses. You can read about doing this on wine forums, and Jeff Sparrow does have a section on barrels in "WIld Brews" that is a good read on the subject. At first, the water rinse was yellow from the oak tannins, then less so. A final rinse with citric acid added (a small %) is recommended to neutralize the caustics in the barokleen. Due to this product having caustics, wear safety gear, including something for your eyes.
Some put a beer into the barrel to finish the preperation. A friend used an IPA, in the barrel for a week When your lambic is ready, take out the Beer and put the lambic in the barrel. Once bugs and critters are in there, that barrel is only for lambics. The bugs and critters will take up residence deep in the wood pores, and can not be cleaned out. This is what gives the barrels at Cantillon their magic.
The barokleen as shown at midwestsupplies (4 lb) and NorthernBrewer ( 1 lb) NAYY.http://www.midwestsupplies.com/barolkleen-4-lbs.htmlhttp://www.northernbrewer.com/winemaking/wine-equipment/oak-products/wine-oak-barrels/barolkleen-1.html