DPM and Alternatives
Chris,
There are a number of alternatives to DPM but keep in mind that DPM kits and panels are extremely good, both in terms of quality and versatility. If you are good with a razor saw and file you can even narrow or shorten panels for yet more variety.
Korber Models (KM) also make some very nice modular panels for brick walled buildings. I purchased some a while ago to test and develop ideas but have been busy with other big kitbash products ever since. These modular panels match a number of their kits so are good for making those kits taller, longer, wider or adding annexes etc.
City Classics has been mentioned and almost all their kits are a great basis for kitbashing. The Smallman warehouse is expecially well suited for warehouse and factory projects as a complete line of panels is available to supplement the kit.
Outside of brick walled structures Great Western Models makes a wonderful line of "pre-Cast Concrete Panels" for more modern warehouse/factory and distribution buildings. There have been several articles on using these panels in the MR press over the last few years. Pikestuff sells a good line of kits and "extention" panels for modern steel sided and roofed structures ranging from small single stall enginehouses to factories or warehouses. While Micro Engineering don't make modular panels several of their kits readily lend themselves to kitbashing. A Micro Engineering machine shop was the basis of the main building annex in the photos I posted under the chain link fencing thread about a week ago. (It is the light beige extention wraping around one side and corner of the Vulcan Manufacturing Plant.)
Similar to DPM are Smalltown USA models. If you didn't see the packaging at first glance Smalltown USA can easily be mistaken for DPM but under closer scrutiny don't quite measure up in window and door details.
I haven't seen the Walthers modular panels yet but they have released brick sheets that match their cornerstone brick walled buildings. (Note that thses bricks are scale sized so they won't match with the older cornerstone kits whose bricks are much closer to O scale. Those bricks have been a thorn in my side during the kitbashed engine house I'm building for my brother right now!)
Almost all kits can be used as "panel stock" for kitbashing. A couple of years ago I bashed a pair of Kibri sawtooth roof warehouses into a double length and widened version on a custom foundation to fit on my brother's layout where a siding was much higher than the surrounding ground level. The extended foundation was about 20 ft high and made for a very unique and centrepiece model. As it was at the front edge of the layout it received a number of interior details to which I will add sometime in the future.
All it takes is some practise with a razor saw and file to trim off pilasters and a few test panels to develop glueing techniques. Buy a couple of cheap kits and play around with them to get some proficiency before "taking the plunge" with more expensive kits. It is extremely satisfying to have your own buildings that aren't duplicated on countless other layouts and which custom fit your urban properties.
Cheers
Carl