Help!! I messed up!

I used Expedia earlier to do a flight itinerary on multiple airlines, connecting in LHR (LAX - MRS). I got a good price, then waited too long.

So now I can either pay a much higher price for the ease of having my luggage checked through booking on one ticket, or paying less but having to collect luggage / customs in LHR by booking each segment individually.

Are there any other booking tools to try to get multiple segments on one ticket? (At this point, don't mind mixing classes of airfare like business on long haul while economy on short haul.) Are there any other tools I can plug in exact flights I want and get a price?

Any advice, thoughts, ideas would be great! Thanks in advance!