There are some posts around for em, their okay and hold a couple more BB's than 30 so the two or three you lose that dont feed in every mag are not taken from the 30 rounds. I've got 20 of the 20 round ones for my G3 and just bought another 10 for my M733 for a total of 20.
Its actually cheaper to get em from
ehobbyasia as the cost after shipping and customs is still less than the cost of them from a Canadian retailer + tax + shipping. Or pick em up from the sale section, they pop up from time to time for cheap. You just have to know that when ehobbyasia says "combined shipping" they dont mean "discount on multiple items" as every time I order the just add all the shipping costs together. If you order a certain amount they also send it EMS instead of regular post.