Let me Google that for you…
I use AWUK a fair amount. Their shipping is quite high if you order small amounts of things, so I usually wait til I have a few things to order. Their eBay store is slightly more expensive than their main store, probably due to listing costs etc.
ArcadeJapan look a little cheaper, never used them.
Maplin sell budget switches that fit, if you’re near a high street, you won’t pay shipping, but the switch will cost the same base cost as a Sanwa and not be as nice, nor as consistent.
Rs Components will sell you Omrons for around the same price.
Not a lot of choice in the UK sadly. I’d just bite the bullet and buy the right part from one of the first 2, rather than nickel and dime over shipping.
Try to keep this stuff in the Q&A thread in future. You’ll get the attention of the forum veterans, who may know better.