I've purchased ours from Barclay Wood Blocks- They are made in the US by a small shop in Indiana. I absolutely love them (the blocks). I've had fantastic customer service too. We bought the "infant A" set first, and have added the "toddler A" set, and a few individual pieces. The big sets are too expensive for me to afford, so I figure I'll periodically buy small sets and eventually end up with a proper building set of unit blocks. I bought one "premium" set and one "seconds and firsts". The only difference is in coloring- the premiums are all uniform light wood and the seconds have some darker bits with some interesting color variations. I am torn- I like the uniformity of the premiums, but some of the variations in the seconds are actually really pretty.
I love Unit Blocks- it's one of the play activities I truly enjoy doing with my daughter. And I know these blocks will still be around to play with the grandkids some day too!
http://www.barclaywoods.com/