Seems you are trying to get the old
libdb4.8 from a previous Raspbian version with oldstable version in
/etc/apt/sources.list But this doesn't help much because you are really very outdated with your installation. You are using raspbian-jessie that is already oldstable. If you want to address the predecessor of jessie you must use oldoldstable, that is wheezy. You can also look at
http://raspbian.raspberrypi.org/raspbian/ if you find
libdb4.8 in one of the older Raspbian versions.
Btw. a simple search with libdb4.8 on google give me over 30,000 matches and just the first one from ubuntu shows where to get the library dev files:
sudo add-apt-repository ppa:bitcoin/bitcoin
sudo apt-get update
sudo apt-get install -y libdb4.8-dev libdb4.8++-dev
Maybe you will find also the library there. If not, then there are 30,000 other links.
And yes, you should really consider to upgrade your software to latest stable versions.