You can buy your balloon and/or have it filled with helium at any balloon shop or bring a one-way helium canister home.
I first determined the weight of all parts and added an estimate for the fin (that was not cut size at that stage) and for glue and cellotape.
item weight in g
3 hacked servos 3 x 1.7
3 propellers 0.17
80 mAh battery 2.6
33 cm rod 0.4
battery lead 0.3
depron fin (estimate) 0.5
allowance for tape and glue (estimate) 0.25
So the concept promised to be suitable for 11” latex balloon (a common party balloon) or any balloon with a lift capacity of at least 10g. Ballast is added to achieve the desired buoyancy and removed again to compensate for the loss of helium over time.
A fresh 11” latex balloon should lift 11g at least (according to manufacturers reference data for 300m above see level), but you will soon notice a loss in lift capacity. In balloon shops latex balloons to be inflated with helium are commonly internally treated with "Hi-Float". This makes them float longer.The one I bought lifted 14 g (about 30m above sea level). I ended up using a 125mAh battery bringing the weight on 11g and allowing for about 3g of ballast. As expected regular trimming by removing ballast was needed and as after about 10 hours al ballast was removed. But that is not bad for a 1.5 EUR balloon.
A fresh 14” latex balloon however carries more than 20g so there is some room to last a couple of days (again with "Hi-Float"). For the camera version I had a 16” balloon with a lift capacity of over 35 g available.
Foil balloons keep their helium much longer (up to weeks and can be refilled), but have a higher proper weight, making the minimum suitable sizes larger. My favorite foil balloon for a micro blimp is obviously the Zeppelin NT as sold by the Deutschen Zeppelin-Reederei GmbH https://ssl.kundenserver.de/s7875334.shoplite.de/sess/utn;jsessionid=154edbeeba05160/shopdata/index.shopscript and lifting about 21g. A nice alternative, more commonly available is the 40” Letter ”I“. Its lift capacity of 18g is quite tight for a camera version, but it is possible without the camera or with an extra lean build (like powering both camera and propulsion from the flight battery, see last step).
A not on the helium: for these small blimps pure (99% or something like that) is strongly recommended as there is little buoyancy to spare. But where gasses like “Ballonal”, a mixture of helium with nitrogen used to be popular, nowadays almost all balloon shops work with pure helium.