My favorite sidelock (flintlock) is way too long for most gun cases. I've got a really good aluminum hard case for when I travel by airline, but it's too big & cumbersome for regular use. I just use one of those silicone impregnated gun socks for mine most of the time.
I just use a cheap-o soft shotgun zippered case for mine. However I've got no scope mounted. I find the Allen brand cases you find at walmart to be good quality. About every case I have is made by them. I simply can't tell the difference between a $15 Allen case, and a $40 Browing case.
Unless of course your going for style.
I bought the $35 horseblanket indian-pattern sleeve or sack or whatever. The Lyman Deerstalker .54 fits OK (caplock model). The soft pachmyer recoil pad gets a little hung up on the internal canvas on the way out, but if you're patient it comes out just fine. The case is not very wide so it just fits. But I really like the look, the cloth is very thick, and the leather tie strap is very nice quality.