I started collecting DDM just for my roleplaying needs, and after the 1+6 boxes I was hooked - if you've collected anything you know how it goes. Anyway, as many DDM collectors know, there has always been some holes in DDM collection, even if many of them got filled close to the end of the line. To fill these holes, you need to find alternative sources. Horror clix, Heroclix,Star Wars miniatures, Mage Knight and Heroscape are main sources for miniatures I've found.
Unfortunately many of these start to run out of singles stores, so finding some of them may need a bit searching. If you really want something I believe they can be found without too huge trouble. So, here you can see some of my non-DDM miniatures I find essential additions for DDM.
Pegasos rider, Vampires, Woman-at-arms, Staff fighter, Stag
Angels (and another vampire)
The best angel -miniature I've found is Dreamblade Angel of sunrise - unfortunately it's Out of Stock at T&T, you might want to google for other sites. If you want an angel for your Invoker of Raven Queen, Thief of Last Chances is a perfect choise. Vampire Enforcer is another excellent vampire (unfortunately out atm)
If nature has a big part in your games, one really excellent miniature is Horned Stag - again one miniature that's been sold out, hopefully you can find it elsewhere if you want one. I could have sword there were many left a few days ago... It's too bad that way too many excellent rpg miniatures are being sold out.
If you need gladiators, em4 miniatures has a game with 5 prepainted gladiators. While I haven't been able to compare them to DDM, at a glance they looked quite same scale.
Heroscape has also a few good-looking gladiators, which seems to be available at Amazon although expensive: http://www.amazon.com/Heroscape-Expansion-Set-Collection-Gladiators/dp/B... . If you find them in your FLGS (means Friendly Local Gaming Store if you weren't aware) for more affordable cost, go ahead and buy them!
Have you checked my series of posts for Miniatures for Sword & Sorcery -articles? They have a lot of miniatures that fit traditional fantasy too. Especially the article about mounted miniatures and commoners may be of interest to you. There are also some non-DDM I've bought especially for D&D games, but I haven't posted them here to avoid repetition.
And in case you're curious, I've cut 1" (or a bit less) discs from 1mm cardboard, glued debased miniatures to them and painted the bases black. I would have painted them dark brownish grey but I have to confess black's so much faster and I'm lazy!