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Find a source of switches will be the least of your worries if you want to build and program your own keyboard.
There is an offer of switches above, but another idea is to go halves with someone in a groupbuy for switches, as such GBs usually provide 100 or 120 switches. If you can find someone who wants 100 switches, you can grab the other 20 and spare postage.
Or, and perhaps this is more difficult for Gateron switches, find an old, broken keyboard on eBay and harvest switches from it.