Not as easy as you would think, most miners are made in China and you need to be buying in wholesale and pre-ordering. Most don’t have distribution channels so you’re relying on Resellers and even then they might not be in your country, so expect Import Duties, VAT, Tax, etc to be added on to the bill.
You also have to be careful as there are plenty of bogus sites supposedly selling ASICs, often with payment methods where you have no buyer protections.
So our options are eBay, Amazon or Aliexpress
Manufactures of ASIC machines are currently Bitmain, Baikal, Ebang, Innosilicon, Futurebit, Halong, Pangolin, Gridseed and Gekko Science.
USB Miners are unprofitable, treat them like mining with your computer, and education and somewhere to start.
Also, something to be aware of, the profitable miners will suck electricity like boiling a kettle 24×7.