You need a plain old straight cut bit. A high speed steel one usually is included with a router, but you want to get the carbide tipped ones for MDF (otherwise it heats up and burns the MDF and dulls the bit I've found)
If you have a Menards in your area, the best deal is a 6 pack of straight bits (carbide tipped), from 1/4" to 3/4" every 1/8" (1/4, 3/8, 1/2, 5/8, 3/4) plus one other size. It is normally $8.99 but goes on sale for only $4.99. You won't find a better deal anywhere!
otherwise if you need tons of bits, there are sellers on ebay with 50 different bits in a box with door for around $40 shipped if you look closely (warning some try to tack on $20+ shipping, most are in the $15 rage, but you can pick up a good deal if you watch). This set, or the 80+ piece one are the sets if you want nearly every bit you'd ever need.
Buying a single bit at Lowes/HomeDepot runs about $10-$40 depending on the bit. I've found the imports work just as well and are as safe.
$5 for a box of 6 straight bits will get you by for a long time, use the money you save to get a good tmolding slot cutting bit.