I want one.
What looks like the best place to get a onesie (for all ages) in the UK is from The All In One Company (They ship worldwide but it costs extra).
Be as babyish as you want, add ears, hoods, tails, mittens, feet, tabs and drop seats, Plus you can choose the zip and if it’s on the back or the front. Add emblems, and custom motifs to boot. The only draw back (aside from the price) is the long wait of two weeks which means it’s hard to guess when it’ll turn up which might be hard to explain for some people.
Sadly they were on holiday when I wanted a onesie and I couldn’t wait so I ended up getting a cheaper, less awesome, onesie elsewhere.