Was in a pawn shop - saw a Piano Acordion with cut up chunks of grey car seat belt strap tied with big messy knots.

When looking years ago for straps to tie down my Symphonia so I can play it hard and fast - one around the neck and one around the waist - I ended up with some narrow bright yellow woven plastic adjustable things with minature brass 'dog clips' on both ends - but they were cheap and do the job very well. I think they may have been some sort of camera strap.

You really can use anything that fits and is comfortable - but it may not always look so pretty. After all, it's supposed to be the MUSIC that counts...