Here are a few suggestions for creating/buying a nicer look for game pieces, depending on how much money you want to spend.
The version with the metal pieces is the 1999/2000 Hasbro(Avalon Hill) Big Box Edition, and it comes with a pretty hefty pricetag (usually $70 USD or more) because of its premium units. A lot of people seem to like to combine the 50th anniversary board with these metal pieces, apparently the colors match quite nicely. You can get this edition, or even the classic Dip editions with wooden or plastic pieces through the boardgamegeek.com marketplace. Lots of european sellers on there.
Another solution might be to find pieces on boardgameextras.co.uk. Looks like it has some nice ships from Seafarers of Catan, in quite a few colors, as well as other Catan related units in the same colors that could be used for armies.
These presuppose that you will get to play it often enough to justify the additional price. At the end of the day, the board could be lines in the sand with rocks for units, what matters most is that you have a group of people to play with who will really throw themselves into the experience. The board and pieces themselves are just a place to watch your meticulously prepared strategy be torn to shreds by your "allies", while they high-five each other and steal all your supply centers and snacks.
If you buy used, here is a link to a site detailing the various versions, along with pictures. It is a bit old, missing the last dozen years or so since its a dead site, but it is comprehensive.
These are the Avalon Hill versions -
http://mysite.verizon.net/~vze4r2c2/Dip ... US-AH.html(at the top of the AH page, is a set of dead links to the other publishers versions. To see them, just replace the part of the full link after .../Diplomacy/ with the same part of the html name you see on mouse-over of the page you want to see. For example, to see the Games Research versions, you would replace US-AH.html in the above link with US-GRI.html)
For what its worth, I just ordered the 50th anniversary edition too. Most people suggest that the cardboard chits are flimsy and feel cheap, so I will probably replace them with something else eventually. However, I'm not sure that I see the problem with trying to fit two pieces on one territory, why would you need to do this at all?