The red undergraduate gowns can be worn at any of the 'ancient' unis: Edinburgh, St Andrews, Glasgow, Aberdeen and Dundee - sometimes with slight differences, for example where St Andrews has a maroon bit at the neck, Dundee has a blue one.
St Andrews is the only place they are seen commonly; although they do still make an appearance at some things in the other four: usually debates, chapel, choirs and so forth.Edinburgh, being very much a city uni, seems to have shunned them. Out of the five that are entitled to use them, it seems Edinburgh is the least likely place to see them. They were fairly common in Aberdeen and Dundee up to the 70s and 80s. Glasgow, I don't know about, but I've heard a couple of die-hards still use them.
If you really want one, then good on you! But you'll be part of a very tiny minority. I imagine the Students' Association might keep some on hand to loan out for events like those mentioned above and maybe a few people will have inherited one, but yes - not at all widespread. If you want to buy one, then the official robemaker is Ede and Ravenscroft and it'll probably cost you just over a hundred pounds. I suppose a couple of folk must go out of their way to buy them.
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Edit: And of course, there's gowns for graduates, just like at any other uni. But I presumed by your question you were meaning for undergrad students. Oh, the Students' Association Executive members will probably wear their ultra-fancy gowns for certain things too.