You mean Yellow-vented Blue Bonnets? Behaviour wise they are the same as Red-vented Blue Bonnets. And looking at the rarity (although more an more birds are coming available due to good breeding results) it depends what you want to pay for them.
If you search the parrots section of the advertisement for "yellow" you may find at least eight breeders offering yellow-vented blue bonnets. Note that there's even a German breeder among them. Contacting these advertisers will probably result in a fair avarage purchase price.