Here is my best advice and how I learned to find my correct sizes.
I started going to Thrift / 2nd Hand Stores. Goodwill, Salvation Army, Purple Heart etc. I would find a blouse or skirt, or what ever I was looking for. Find it at a good price, then buy it, take it home and try it on. If it fits, great. If not, you know if you need to go up or down in sizes. I did that until I figured out my sizes. I did that for bras, blouse, skirts, dresses, pants etc.
Now that I know my sizes, I STILL go to those Stores and look for bargains to buy for my wardrobe. I bought a nice new skirt there yesterday !
Yes, it might cost you a couple dollars in the beginning for something that does not fit, but if you shop right, it's not that much money. Plus, you can always donate the clothes that do not fit, back to the Store and write it off on your taxes. That's why I recommend the Thrift Stores.
Or, if the Stores have changing rooms you can always try them on in the Store. If you want to be discreet while doing that. You can find the women's clothes you want to try on, load them into your arms, then throw a men shirt over them, so it appears you have a arm load of mens clothes. Then go into the dressing room and try them on.
The only warning I have is it can be very addicting.
Enjoy,