I rapid prototype several new sensor and hardware concepts each week. My productivity at the concept stage for medical sensor devices leans heavily on open source platforms like Arduino and Raspberry Pi.
Open source hardware hubs like SparkFun and Adafruit are always on a techie's radar with their easy-to-use, well-documented examples and active developer communities, plus instructional videos, project ideas and tech support. These are great places to get started and to challenge yourself with new projects.
Adafruit's NeoPixel products and code examples are fun way to light up your projects with inexpensive LED color strips.
I also like the inexpensive and versatile 3.3V 8MHz Sparkfun Adruino Pro Mini microcontroller boards. The Pro Mini is useful for programming a device that works by itself and doesn't need to talk over serial to a computer. 3V operation means you can talk to most low power sensor ICs over I2C without a level shifter. Without the on-board USB connector it's a little cheaper than options like the Nano or Due. Don't forget to order the FTDI board, or you won't be able to program it. (Avoid the buggy Pro Micros, though.)
China based and dirt cheap, UCTRONICS may have what you need at a fraction of the cost of a US-based supplier, like Arduino Pro mini's for $2.38! (vs. $10 at Sparkfun and Adafruit). If you can wait a few weeks, UCTRONICS will be your lowest cost source of sensors, development boards and microcontrollers, plus they source sometimes hard-to-find hardware like Arducam's NOIR camera module for Raspberry Pi with infrared sensitivity. Be sure to stock up on jumper wire and headers, but don't expect any documention for development boards! You are on your own--but at least you have some extra money left over in your project budget.
MPJA.com is a US-based supplier of hardware and tools. Pic up cheap power supples, soldering irons and other accessories. If you save up and get an order over $50 or $100 MPJA often runs specials and you can land yourself a free multimeter or cordless solder iron--pretty sweet. Faster than UCTRONICS, but not as rock bottom cheap, head to MPJA when you need it faster than snail slow, but still want to save a penny.
DX.com has much more than just hardware for projects. Get kits, gadgets, personal electronics and other geekware for dirt cheap. Fast delivery, low prices and so much to choose, from robot parts and apparel to DIY supplies and car accessories--even makeup (!?). With roots in China, but US stock, DX is like a geek-worthy overstock warehouse with stock products, too--truly the best of both worlds. Fair warning: if you spend all your birthday money at DX on drones and drone parts, don't blame me for not warning you!