Interesting question.
Handler or friends is one solution.
Only bring things you can't get in a store plus a small phone, credit card or some spare change. If your regular phone is a tad big, then get a cheap small backup without sim card. Most of it can probably be hidden in the footpads, head or carried around the neck if you got the space for it. But I guess our more experienced suiters can answer on that.
Or just let your character carry a bag or backpack.
NoxTigress' suggestion isn't too bad either.
The last time I went fursuiting was at a park. I packed a duffel bag full of picnic supplies, my camera, my phone, and other things. My two spotters were more than happy to fetch things for me, but having a bag-like thing to put them it came in handy.
I'm going to an anime convention in May, and I plan to wear a small backpack of some kind so that I can keep my room key and stuff in there.
Yeah but you see I can of have issues with having stuff not actually on my person. I don't like back packs either ( people can grab stuff without you noticing, especially in suit) I guess I could find an in character tote bag that I can have on me.