Chastity Devices

So my mistress as expressed as desire to try out chastity. This is something completely new to me because before her I would have never agreed to this, but alas I can't say no. So I am coming here to ask for help. Anyone have suggestions on what to buy? Where to buy? What to avoid?

If it matters at all I am a grower so while soft I am fairly small I grow significantly once aroused.

Any help would be extremely appreciated.