MarHyde Ultimate 2K urethane high build primer. It's about $100 for a gallon with the hardener. No need to spray it, use a small trim roller and roll on several coats then sand. Pot life is about 3-4 hours, so plenty of time for it to flash in between coats. Keep in mind that this is an isocyanate based primer and you need to have good ventilation and a proper filtered respirator.

Another good high build primer is Evercoat brand DuraFill primer. It's a polyester filler that is very reasonable cost and has no isocyanates. It's also available in quarts.

http://www.evercoat.com/productDetail.aspx?pID=261

If you don't want to go that route, the SEM High build primer pictured above is a fairly good substitute. Doesn't have the filling abilities of the MarHyde or the Durafill, but in a pinch or for small projects, it'll do.

Both of these are available from most auto paint stores. Some other low cost brands are Transtar and Dupont's Nason.