In general try to stick to the larger distributors or suppliers like Mouser, Farnell (or CPC in the UK), RS Components, Digi-Key, etc, or your countries closest equivalent assuming you have good consumer protection laws, if their target market is the business and industrial sector but they still sell to individuals it will likely be a good bet if a little expensive.

Another good choice can be hobbyist or educational sites that have some reputation to uphold, the most talked about online will be US centric like SparkFun or Adafruit but there's many European equivalents for example. One way to find them would be to find an authorised distributor in your country for larger projects like the Raspberry Pi and see if they sell the components you need. Chances are if they're an authorised distributor for a recognisable product then they're probably not going to risk losing their spot on that products website just to sell some fakes or dodgy components.

The key thing is to not buy directly from China or random eBay sellers from abroad if you can help it, return postage to China is prohibitively expensive and Chinese stores or marketplaces like Aliexpress won't always agree that you're entitled to a full refund without returning the item even if that item happens to blown up in your face, and likewise you can't really expect that small random foreign eBay sellers are going to be liable for selling you items that adhere to your countries safety regulations for instance.