Sorry, but I have never seen "customs approved" tulip bulbs in Turkey. Anyway, the tulips you will see in Turkey are actually the Dutch hybrids that are the modern tulip--the original Turkish tulips are called "botanical species" by nurseries, and you can still get them here in the US today online.
Spices can be brought back without a problem--don't forget to declare them as "food"--I've done so several times without an issue. But stay away from the Turkish "saffron" that seems so cheap--it isn't saffron at all but actually the dried heads of safflower--no taste and a little washed out color. Real saffron is deep red (not orangey color) and comes from Iran. Some of the spice market vendors will have it if you ask, but almost all of the labelled "saffron" isn't worth even the cheap price charged. And what is sometimes called "Indian saffron" is turmeric--which is fine, but easily obtainable in the US and not worth bringing home. If you like spicy food, the Aleppo pepper is a hard-to-find moderately hot pepper, sumac is a tangy spice used in eastern Meditteranean food, and lokoum--or Turkish delight comes in a dozen or more varities and makes a nice gift for friends back home. Also, the "apple tea" beloved of rug sellers will bring back memories for you...
Turkish ceramic tiles are a nice, non-rug souvenir. See the exquisite tilework at the Topkapi palace--especially in the harem section--and you'll see what I mean. If you drink coffee, bring back a cezve, the handled ladle used to make Turkish coffee. Or some of the lovely tiny tulip-shaped tea glasses. And everybody needs an "evil eye" glass amulet to ward off bad luck...Have fun shopping in Turkey--and don't forget to haggle!
