I have a kitchen aid stand mixer KG25HOXWH.

Hi ZPAORUSKI. I suspect your switch problem has to do with the speed control assembly inside your mixer’s head, and yes you can order and replace it.
The inspection and troubleshooting of the inside of your mixer is a DITY capability, particularly if your mixer is out of warranty. Just be sure to have the Parts List manual on hand. If you're lacking one, you can download a PDF version from the KA website: http://www.kitchenaid.com/flash.cmd?/#/page/home. Click on Customer Care>Locate Manuals & Guides> Enter the 6 character model number, in your case: KG25H0 (use zero and not ‘O’), and select one of the last two files listed under Parts List (9) (check to ensure this matches your mixer model. KA made a -4 series and a -5 series with slightly different insides, so make sure you have the correct Parts List manual.) For a series -4 mixer, turn to pgs. 2 & 3, titled Case, Gearing And Planetary Unit that shows an exploded view for the disassembly and reassembly of your mixer, along with all the parts. Locate Illus. No. 8, Control Assembly. This is where I suspect your problem lies. (For a series -5 mixer, turn to Pgs 4 & 5, and locate Illus. No. 18, which is the speed control assembly).
First inspect around the controller assembly to ensure that all the wire connections are OK. It's possible a connection has come loose. Unfortunately the whole controller assembly is fabricated on a little PC board filled with resistors, capacitors and other electronic parts, which was not meant to be 'home fixed'. My 'expertise' ends at being able to recognize a resistor from a capacitor, but I don't get much deeper than that. A local small appliance repair shop should be able to determine if the controller assembly is functioning properly or not.
As far as parts go, here are four potential vendors in no particular order: RepairClinic.com; mendingshed.com; searspartsdirect.com; partstore.com. Prices for the same part can vary widely between vendors, so do your comparison shopping.
Once apart you can check for any other unseemly wear and tear on the gears, etc. It can be messy but quite gratifying to diagnose and repair your mixer yourself.
If you feel troubleshooting and repairing is beyond your capability, then I would seek out a local small appliance repair shop in your area. KA Customer Care can give you names of local KA authorized repair shops in your area, but you can also do an online search to supplement what KA may provide you. Call to explain your circumstances and find out their hourly rate and get an estimate of what they think it would cost to repair your mixer. Also, always obtain three or more estimates – do some comparison shopping. Good luck. I hope this helps. Howard, Burke, VA
If this solution has helped you, please rate it, thanks! - hslincoln