love2sleep - If you're down to one feed, then your size really won't change very much if you stopped any time soon, so I wouldn't worry too much about not getting new underwear - it would fit fine I think.
When my two stopped, ds just lost interest over a week or so. It was always part of the bedtime routine and he would always have asked for milk, but eventually it was me asking him instead of the other way around.
He'd had milk in a cup in the afternoons for a while and just asked for milk in a cup one bedtime and that was it.
With dd she started becoming more fidgety and distracted at bedtime and just wasn't bothered. I offered her milk in a cup and she wasn't interested. So one night I said daddy was putting her to bed, thinking she would scream the house own demanding her milk, but she didn't. I was half gutted and half happy that she was moving on to another stage. I had hoped to get to the magic 2 years as it's so good for their immune system and neither of my 2 have been great milk drinkers other than breast milk.
Interestingly now ds is 5 and dd is nearly 3, they're suddenly very keen to have a glass of milk nearly every day - go figure!
Anyway, I thought 'self-weaning' would be some dramatic drawn out process, but actually for us it just meant they lost interest and were quite happy to do without. So we stopped.
With baby no3, I'll do the same, but will definitely try to keep it in their diet for as long as possible - for their immune system and purely selfishly for me for all the health benefits I'll get from b/f long term.
Go and treat yourself to some lovely undies and carry on for as long as you feel like it