There is a fertilizing section in every plant file in Yardener's Helper. There is some differences between perennials, annuals, and bulbs when it comes to types of fertilizer. Fertilizers designed especially for flowers tend to have a bit more phosphorous which is needed to help encourage blossoms.

You can find a fair number of options in the garden center:

Slow release granular fertilizer - There are some fertilizer products sold for perennials and annuals specifically, but for the most part you will find a general purpose granular fertilizer is the option for flowers.

Timed Release Fertilizer Pellets - These products are designed to release their fertilizer over a period of 8 to 16 weeks or sometimes even more. While they tend to be more expensive you don't need much to do the job.

Liquid or Water-soluble Powders, - These fertilizers are used either as a foliar spray or as a drench directly into the soil. You can find synthetic products or organic liquid fertilizers. Many serious gardeners use these products in a very, very dilute form evey time they water their flowers. This can be effective but if over-used, serious problems can develop among the plants. These products are used as snacks in many cases.