If you’re buying new:

• Musiciansfriend.com
• Sweetwater.com
• Guitarcenter.com

Some of these offer financing options. Or just walk into a music store to test/buy.

If you’re buying used:

• Offer Up (or similar apps)
• Facebook exchanges
• Pawnshops

Just make sure you research what you’re buying so you don’t get a lemon.

Other conventional wisdom of mine:

• Guitar/bass: buy commonly known brands, invest in a decent amp
• Drums: spend just a little more, beware of plastic parts that can break
• Keyboards: basic Yamaha keyboards give you a good bang for your buck