Ebay is a start. There are 15 listings currently on ebay -- just do a search for casino chips from monaco monte carlo.
Check the sold listings for prices too, so you don't overpay. While some pieces can be harder to obtain, if you're only looking for a few, you'll find them at reasonable prices.

Also the french ebay site (ebay.fr) usually has some listed too, if you don't mind dealing with international shipping.

If you want to check out the different chips and jetons available, there's a pretty extensive listing at the webpages linked from this Chip Guide page:
http://www.chipguide.com -- then search for Europe / Monaco in the drop-down boxes.

Finally, there will surely be some for sale at our annual chip convention in Las Vegas in 2 weeks -- South Point Hotel-Casino.