Like Jeanie, I'd been looking for shrubs too. One that I'd checked out was the osage orange that Janet mentioned but I'd rejected it because I couldn't see thorns on the twigs. According to my tree and shrub field guide Maclura pomifera has "strong unbranched thorns at each leaf scar......This species and the bumelias are the only thorny plants with milky sap." Edited to add that would be in North America of course. Since it came from an arboretum it's not necessarily going to be in a North American field guide because its origin could be another country.
I just noticed the arboretum has a Maclura pomifera cultivar 'Altamont' which appears from limited Google results to be thornless - no idea if it comes true from seed. The buds should be ball-shaped rather than pointy though.
