Making something to blow up, huh?
No need to buy it, even if you could.
Methane is just natural gas (NOT butane or propane). Many people's homes already have it. But, it's under a little bit of pressure and, also, it's not supplied in a way that makes it easy to use other than in an appliance. It's very flammable and can be toxic and explosive. It's heavier than air in case you want to try to float something with it. That won't work. Also, since methane is odorless, the natural gas in homes has been treated with a sulfur compound to make it smell bad, so people can tell if they have a gas leak. So, it stinks. Just be careful.
**correction** I was mistaken and got a bit ahead of myself, haven't had the third cup of coffee yet; methane is lighter than air but, propane and butane aren't. It's still not a good idea to use it for flotation, though. It's flammable. And, from when I was a kid, I don't recall methane being an efficient LTA choice for small balloons made from heavier material. Might do better in mylar ones, though. Didn't have those when I was young.
**Just a thought** I don't know the nature of your experiment but, was just wondering if you've ever tried collecting methane off decaying plant/animal material? Might be fun and interesting. Can't hurt to go 'green' in your experiments.