Stanley,
You have many options.
1. Buy reproduction grips, these can be bought at Sarco, Numrich Arms, Tom Heller probably has some (link to your left at
http://www.lugerforum.com/lugerlink.html
2. Buy repro from e-bay or other gun auctions sites
3. Buy real old ones, they can get expensive
4. Hugh Clark finishes some of the repro ones and makes them fit perfectly, and this is a slight concern, many of the repro ones don't fit worth a darn.