Hi and Welcome to TWF! Glad to have you here.
I hate to welcome you in one sentence and warn you in the next, but feel like I must. Branded clothing is one of the most counterfeited items there are. You must be very careful when buying.
And they are also very low profit, as are most branded goods in general. If you are planning on selling from a shop in the Czech Republic, fine. You may do very well.
If you are planning on selling online through eBay or otherwise to the entire world, you will not make a lot of money at it, as it is a very competitive field.
Having warned you I will tell you that no one here is going to just give you their suppliers. We will all help you in other ways, such as shipping, site building, importing, etc. But no one gives away sources.
You may find some, and probably will, in the membership site I promote in the top link below. I make no guarantees, but I do know there are many clothing suppliers listed. There is a dedicated section for them.
BTW, the price of membership is going up on Saturday March 1. It is still Friday night for me and I'm too tired to change it tonight, but some time on Saturday it will go up $ 3.00.
Whether you join that site or not, I do wish you good luck. It's always good to get new folks on TWF!
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