Just when I thought my STP packer was gone for good, I found it in a hygiene bag (with a bunch of tampons coincidentally, haha) under the sink. The problem I had with it though was that the tube opening wasn't big enough but I bought the STP from an independent seller and figured it'd be much more efficient just to ask you guys where the best/cheapest replacement tubes might be bought?

If you just want tubing aquarium tubing is very inexpensive. I don't know you have to know the inner and outer diameter to these things.
Hardware stores like Lowes also carry it. Some of the stores will have a bunch of different sizes and you can select some of each. Might have to buy it in feet, I don't know, but it should be cheap.

--Jay Jay

Yeah I agree
Sometimes places have off cuts so you might not have to buy it in feet just ask

Thanks. The main thing really is I'd like to invest in a larger opening but I was looking at it last night and it doesn't look too complicated to make, just have to figure out what kind of glue to use when putting it all together. Any suggestions on durable one?

Try loctite 424 instant adhesive it's made to glue rubber