amiami generally have the best price and very reasonable shipping costs (I pre-ordered MP Prowl and Bluestreak from them for 4550 yen each before shipping) But popular items sell out incredibly fast due their relatively small order quantity.

HLJ and Hobby search are also reputable online retailers. If you are looking at pre-ordering MPs Japanese sites are always the cheapest. But if you are after already released figures the chances are slim that you can find one on the Japanese sites. BBTS and RK would be better alternatives.