In that case you need to look for slimline single sided 64 Mb EDO Ram, double sided 64 Mb will not fit, just like 128 Mb modules as there are no single sided 128 Mb modules, at least not as far as I know off ...
I don´t have any single sided 64 Mb Simms and don´t know any private person that can sell them but I think there must be several shops that have plenty of those to offer :-)