I am new here my Girlfriend also my mistress told me she wanted me to buy a male Chasity device on my own and put it on and lock it and give her the key and become her sissy, which I am ok with that but I do not know what kind of device to buy or where to get it or how to tell if it will fit, I am a bigger sissy about 300lbs and about 5'10 any help would be great she gave me 1 week or else.
dhgate, eBay, Amazon, aliexpress are all online sites you can look for devices on. Just know that your first cage will probably not be perfect. For a first timer look for something that comes with multiple rings.
May want to go to hardware store and buy some different size metal rings. See what you can wear for 24 hours straight to get a good idea of your ring size.
Contact Mature Metal, they make Metal custom cages, which are not insanely expensive. Order the sizing rings, then follow the directions on how to measure yourself. I’ve bout several CB series, a custom bet and the MM has been the best hands down. Plus if you hate it the resale value is decent. After that you’re getting into serious money. I disagree with the suggestions above as the cheap Chinese ones IMHO aren’t worth the hassle. Even with measurements I had to send my MM Jailbird back for adjustment and having a company that backs their products is nice.
The one week limit is going to be a problem. You are almost going to have to start with one of the knock offs on eBay. There is a seller called "Chinese girl Helen" that I have dealt with. She is honest and ships fast. Depending on your length the best bet to start would be either a CB series of a Holy Trainer copy. I would suggest the Holy Trainer as the rings are more comfortable than the cheap versions of the CB's. They are also a very faithful copy of the original as to fit and finish. I tried the Standard and Small versions before deciding that the small was the right fit for me. One caution. The lengths are to fit you flaccid. Most everyone seems to think they are bigger that they really are. Your penis should be in contact with the entire inside of the cage/tube 95%of the time. Has she said what the 'or else' will be if you don't meet the deadline?
Amazon has cheap ones, maybe buy a couple with multiple rings, overnight it if possible. Try em on before giving her the key or at least have some arrangement if the one your wearing is no good. First time and full time could be disastrous so be careful!!