I got mine at mothercare. Places like ebay and amazon also sell them. I've only used them once mind you- her nails were very soft and just snapped off at first, now at 7 weeks she's big enough to cut them.
DD2 is 5 weeks -never used them on either baby including the integrated ones. babies use their hands to explore from birth - it's important for their development of their motor skills. I just use a little baby emery board and baby scissors to trim off excess.
only have one little scratch so far and if disappeared in a couple of days.
The midwife at my antenatal classes advised not using them. As others have said, their hands are how they explore the world. Also their nails are so soft that just them grabbing your top etc will be enough to keep them worn down and short - if you put scratch mitts on then they will get long and sharp.
Mothercare definitely do them (that's where we got ours), or some supermarkets may do them. Though our lb soon worked out how to get them off. (Hence why we used the babygrows with attached ones as much as possible)
Don't bother you won't need them, I never used the ones I bought they just fall off, and only uses the foldover ones to keep baby's hands warm outside! Their wee nails will just peel they are so soft or agree a nail file is much easier