For a beginner, finding the right parts and equipment to purchase can be overwhelming. Conducting a simple Google search for parts can reveal a plethora of websites, many of which are targeted toward experienced engineers, as well as confusing data sheets. However, if you know where to look, you can find some very useful hobby electronics vendors that make shopping for parts easy and can also inspire you to form new ideas as you browse through parts listings. Below I recommend a few hobby electronics vendors I frequently use as well as some industrial electronics vendors I use when I need something less common.
Parallax is the creator of the Propeller microcontroller, so they are generally the first stop for all Propeller products. The sensors, robotics and human input devices sections also have useful parts.
Sparkfun is a very popular hobby electronics website. Although they carry relatively few Propeller microcontrollers, Sparkfun has many useful sensor and robotics parts. At the moment, Sparkfun is my favorite vendor of hobby electronics as their selection of items is so large and diverse. Sparkfun also has useful tutorial information and great customer service.
Pololu is a similar to Sparkfun but does not have as many items. However, Pololu carries many useful parts that Sparkfun does not stock.
Adafruit is another hobby electronics website like Sparkfun or Pololu. Again, the selection is not as diverse as Sparkfun's selection, but Adafruit carries some unique items.
ServoCity is another hobby electronics website. Although they offer a wide range of electronics, I find them most useful for their line of modular structural components. If you want to build something robust and modular, ServoCity's line of Actobotics components cannot be beat. Almost my entire open-source syringe pump design was created around Actobotics components. ServoCity has quickly become one of the first places I check when I am building something new, or even thinking about how I might build something.
Futurlec is somewhat of an intermediate between a hobby electronics vendor and an industrial electronics supplier. Their website is not as user-friendly as the websites of Parallax, Sparkfun, Pololu and Adafruit, but they still carry some useful parts. I find myself shopping at Futurlec more for boards, wires and connectors than sensors and robotics parts.
Jameco Electronics is an industrial electronics vendor with some hobby electronics parts. Their selection is not as large as other industrial electronics vendors, but their website is a little easier to navigate. At Jameco Electronics, you might be able to find that one extra integrate circuit not carried by a hobby electronics vendor and also find the hobby electronics items not carried by other industrial electronics vendors.
Allied Electronics is an industrial electronics vendor. They have an enormous selection of parts. Although I think the prices are quite good, because they also carry expensive specialty industrial products, you might think the prices are not correct at first glance. Browsing through their selection can be overwhelming. I recommend shopping here if only once you know the specific part or type of part you are looking for. When shopping at an industrial electronics vendor like Allied Electronics, you will also need to be comfortable reviewing data sheets. Shopping here may take a little more effort, but the prices and selection are well worth it.
Mouser Electronics is another industrial electronics vendor that is very similar to Allied Electronics.
Digi-key is an industrial electronics vendor similar to Allied Electronics or Mouser Electronics, but I do not find their website to be as user-friendly. As with the other industrial electronics vendors, shopping here will take some effort but it will be well worth the time.
McMaster-Carr supplies almost any construction materials you could possibly need. Although they do carry some electronics, McMaster-Carr is most useful for things like enclosures, raw materials, and construction tools. The website is very easy to navigate so it is worth taking a look at McMaster-Carr when deciding how to build a prototype or final version of an apparatus.