I know all the on-call midwives carry G&A, and they are the smaller ones you mentioned only for about 40mins. But some do carry more, and it should only be a phone call to get more. Most like to have 2 midwives present so the second midwife should have some more.
You shouldn't worry about running out! It's the MW's really will try to make you as comfortable as possible, and if you feel you need it - use it! It is a great distraction!
You are usually at home with one of MW initially, and then when you are about to get into full on 1st stage and 2nd stage labour, they call in for the second one - that should give them lots of opportunity to get more for you. There should be plenty of time, it could be a while, as I'm assuming this is your first labour?
I was lucky enough to have a MW present who had 6 canisters in her car - she must have been a hoarder, like me!
This on-call MW came from her house to me, but the second was called in from the Birth Centre, so they bought all the other equipment they might need - such as a potable incubator of all things (they are really very well trained, and totally prepared for anything!)
If you're concerned, bring it up with your MW at your next appointment.
For me the best time to use it was at the end of the 1st stage and beginning of second stage. you will know when a contraction is building - breath it in, then and breath it out through the contraction. Though again, more as a distracction.
I didn't find it useful or needed at the end of labour. Once your body full takes over, it's just not a matter of pain or even thinking about anything!!
Like Jenstar - I wanted it more for stiches.. that was painful!
But I won't be pushing hard this time round - I really didn't need too.. and blame myself for pushing so hard!
xxx