Practicing Transcription (esp. vowels)

I'm taking a formal linguistics class, and I'm having a hard time with my IPA vowel transcription. Does anyone have some good recommendations for websites or ways to practice the transcription, outside of cuddling up with a dictionary, randomly picking a word, and then comparing?

#edit: and I laugh because I swap British English & American English spellings all the time, and did 'practise' in the subject line but 'practice' in the text. Corrected for American English, since that's what I am.