If you don't want to trade your Mareep, you could add in one of these commons along with the Burmy for the trade.
Seel (base)
Staryu (base)
Corsola (any neo expansion, not sure if there is a common, though)
Phanpy (neo genesis only)
Surskit (any)
Nincada (preferably EX R/S, but anything else is okay)
Gulpin (same as Nincada)
Clamperl (any)
For these, any kind. I am not sure of their rarity, so there may be some uncommons.
Kricketot
Pachirisu
Finneon
Snover