Buy bear-shaped equipment. When not a bear, you will need to tie the very loose clothing and items to yourself with rope that will burst when you become a bear. Be "wearing" it while not an awesome bear, although due to the absurdly poor fit it will probably not benefit you, but this way when you transform you fill it out nicely, because you retain equipment you can use in your new form, IIRC. This is cheaper than magic items, but requires some careful thought as to equipment (hint: cloth everything, inflexible stuff is hard to tie on), and in a very long term campaign it's theoretically possible that the cost of new rope for every encounter will outstrip the cost of magic items for all your stuff. I doubt it, though. Your DM may not allow this on the grounds that it seems silly at face value, but there are probably magic items out there to help (wilding clasps don't, I think, because they're specifically for wildshape, which this isn't, but there is probably something that works for polymorph, or I could be wrong, and your DM should really allow them to work anyway because it's not exactly gamebreaking).