Buying a mining rig online is really risky, as an ASIC miner is really expensive, a standard mining rig usually costs for about 1500 USD to 2500 USD. So the safest website to avail an asic miner is thru the own website of the asic’s manufacturer.
Antminer – http://bitmain.com/
Innosilicon – https://www.innosilicon.com/
Gominer GMX R1 – https://gominer.co/
Avalon miner – https://canaan.io/
You can also avail on other websites like Amazon and Alibaba, or Ebay, but do check the seller as there are some fake sellers on theses websites.