Probably you mean v3.5.0.
The easier way is (if your architecture is x86_64):
wget http://dl-cdn.alpinelinux.org/alpine/v3.5/main/x86_64/apk-tools-static-2.6.8-r1.apk
And then unpack the downloaded file:
tar -zxvf apk-tools-static-2.6.8-r1.apk
You'll find in the unpacked apk the apk.static file, that you can use to install apk-tools.
More info here: https://wiki.alpinelinux.org/wiki/Upgrading_Alpine
Of course, if your architecture is x86, the url is:
wget http://dl-cdn.alpinelinux.org/alpine/v3.5/main/x86/apk-tools-static-2.6.8-r1.apk
Anyway, the package you need is "apk-tools".