for a free option: if you have threaded presta stems get an extra two of those knurled little nuts that hold the stem in place so you have four total. thread one on all the way before pushing the presta stem through the schrader sized hole (inside the rim) install tube / tire normally. now thread on the extra knurled nut from the outside and the two of them effectively sandwich the valve tight on the rim.

i've done this with our two touring bikes and mountain bike and it works great. lots of miles on all of them and no problems so far. got tired of buying / carrying different tubes for all our different types of bikes. plus i can keep my portable lezyne pump set to presta and never worry about flipping the nozzle.