You can get spaghetti squashes at farmers markets. If you do an internet search for your local farmers market I'm sure you'll find one close to you. That's really the best place to buy them as they're a bit difficult to find. I've gone squash mad myself and have searched high and low and bought all sorts and I found the best selection of squashes at farmers markets. You can also get them online from Abel & Cole veggie box delivery service.
My sister has just bought a spiralizer and LOVES it ... courgettes, butternut squash, marrow etc look so pretty! I have one on my Amazon Wish List lol.
I've not seen them in the shops, but my Grandad used to grow Spaghetti Marrows when I was a child (which as you can tell is before the word 'squash' meant anything other than Robinsons Barley Water ). As I remember they were really easy to grow - I might just look into getting some seeds for next year (not much help I know - sorry )