Try looking online for sources of small specimens in quantity and not those being offered for hedging per se.

Typically not sold by local nurseries in those smallish sizes for that purpose due to lack of market - most folks want single (or a couple) larger, more specimen type trees. European style pleached tree hedges are just not at all common here.

You might try a special request with your better local nursery (or one you know that carries a good and diverse tree selection) but most retail nurseries place their grower orders in fall for late winter (like now!) delivery so may be a difficult quest for this growing season.

As noted above, online sources may be your best bet.

You can get the copper beech for 6 bucks each at Burnt Ridge but I don't know how many they have.

http://www.burntridgenursery.com/Beech-Trees/products/70/

Likewise they might have all three at Forest farm - I haven't checked - but almost certainly do not have any of them by the hundreds. And they will be potted. But do notice that that is two places right off that stocks items like this - you should be able to line up what you need by searching effectively.

https://www.forestfarm.com/

I did check Forest Farm. And thanks for the tip about Burnt Ridge. I just ordered ten copper beech-lets for a try at a hedge.