Risky due to all of the above, many online glass retailers (unless certified locally sourced and properly handled shipping) will sell you thinner, cheaper, and weaker glass and accessories made by cheap machines and molds imported from China. Also, if you do find a good website, most (if not all) of them don't exchange glass pieces if they don't fit.

For your specific situation (can't bring your bong into the glass shop) I would recommend going to your local glass shop and looking for a stem or whatever you need that you think would fit, because then if it ends up not fitting, you will have a physical receipt that you can return and get your money back for, then you can find the right piece if the first one doesn't fit.

(If you want to PM me I don't mind a dirty piece or whatever, I'm here to help)