A couple of years back Neotacha and I had the opportunity to visit the National Museum of Scotland (a wonderful museum), which owns 11 of the Lewis pieces. The gift shop sold chess sets modeled on the Lewis pieces, several different price points. But the shop also sold some individual pieces (check the shop under gifts):
http://shop.nms.ac.uk/collections/giftsMost of the original pieces are displayed at the British Museum in London. I expect that institution also has some in its gift shop.
I notice some chess sets sell in the $60 range on Amazon, so depending on how many you want it might be preferable to buy one of the cheaper chess sets.
-Michael