I don't recommend any type of commercial trees... They just don't look right.  I prefer the root method, where you dig out interesting types of roots from shrubs and bushes and then add foliage to them, or leave them bare for winter-time/dead trees...

For foliage, I use a mix of different dried herbs like oregano, thyme, along with static grass and shredded foam.  I also use certain types of branches and such for deadfall, and also the wire armeture method..

As for the types of trees in the former DDR, can't help you, but you should be able to find some pics if you google image Fulda Gap...

http://images.google.com/images?hl=en&q=Fulda+gap&btnG=Search+Images&gbv=2

In the terrain the Russians advanced through, it'll be mixed deciduous (ie, evergreen and leafy trees), and deciduous forests.  In the Baltic areas of the North German Plain, which extends from the Netherlands and the North Sea coast across Jutland (Denmark), birch forests are common, but you'll find maples, beech, and ash as well.  Evergreens will become more common as you move upland, ie, from north to south, to the higher elevations.

As to the trees to use, can't say about the makers you mention, I buy scenery pieces at the home improvement stores, like Home Depot and Lowe's, and at WalMart and Target, at Christmas time, especially immediately after Christmas, when they want to unload the leftovers.  Also at the craft stores, like Michael's, and the dollar stores.

Hope that helps!

Brad

Scott,

Unless you are modeling and painting to a level consistent with the minutiae of detail that would reflect the inclusion of species specific trees, close is close enough. Some deciduous looking trees and some conifers would certainly do the job of representing central and eastern europe. You have to ask yourself, how much is too much detail relative to the rest of the elements that make up the diorama as a whole.

There are some companies that make laser cut leaves that are species specific but these can be pricey

http://www.milminwh.com/mod_scene_leaf.htm.

Since you mentioned that price was an issue I think that with some creative painting techniques with trunks you can no doubt do some birch trees if you so choose and the rest is easy and handily done with materials from the backyard or the craft store.