I don't recommend the century c-836 as it is a pain in the ass to install due to the rough inch of precompression it has (i cut my hands and bruised my fingers)
Is there any particular reason why you don't cut the spring to a more suitable length? If it is because of a lack of tools, bolt cutters or a dremel will do it quickly and easily; most hardware stores will even have a store use pair of bolt cutters you could use for free.
I haven't had much luck thrifting either. Value village was decent for me as they don't seem to value nerf stuff at all so unlike some other stuff you can still get a good price on blasters. Don't expect to find anything, just be happy if you do.