Industrial Paint and Plastics
Oh wait I've said that already.
They might not have "Weldon" brand #3 or #4 (I thought they did but oh well maybe not anymore) but if it's the runny stuff you're after (you need an applicator bottle with a needle) they should have some generic Methyl chloride. I always just use the #16, it's the consistency of model glue so easier to work with - I find the runny stuff (#3 or #4) you need a practised hand (and cleaner edges) to do a nice job. It will make a cleaner seam if you do it right - but spill any at your own peril sort of thing (ie., will put a melt/burn mark in the acrylic).