Buy only from reputable sources that list bulb size. Nurseries categorize bulbs into numeric or alphabetic grades, but look for measurements as well. Larger bulbs produce larger plants and showier flowers. However, you may choose to purchase smaller, inexpensive bulbs for mass plantings.
Some discount sellers simply list common names on their websites, which makes a difference when buying certain plants. Bluebells, for example, may be the tender bulbs of Scilla peruviana or the hardy bulbs of Endymion hispanicus. Always look for a scientific or botanical name listed next to the common name.
Many, but not all, online sources sell and ship bulbs at the appropriate time for planting. Consider the shipping time frame and whether you will need to store the bulbs until the correct season. Also look for guarantees and time limits on returns.