These are the most popular type of cage for hamsters; with a plastic base and a wire frame they offer a secure home for your pet. You can buy these types of cages from most retailers and as long as you follow the sizing guidance on the cages page, you should be just fine. Online retailers often sell these sorts of cages at fantastic prices, but be wary as some companies deliver broken or damaged cages. Make sure you know your rights and don't be afraid to refuse the cage.
Bars should be very close together if you are thinking of keeping a dwarf hamster, they are escape artists and will fit through the tiniest spaces. You don't want your new friend to go missing so be cautious of the gap between the bars. You may be better with either an aquarium or a bin cage.