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The awesome dungeonmorphs are free in multiple formats! These are old-school in art style but made to match each other perfectly in terms of entrances and exits. I have the dice and the cards and love them. You get 90 of them in PDF, PNG, and dungeonographer (which is also free) formats. http://www.dungeonmorphs.com/battlemats.shtml
MapTool long predates Roll20, and while I won't recommend it as software (and it appears to no longer be actively developed), there's a large library of tiles (free and otherwise) that have been built up around it, which should all be compatible with Roll20. Here's a few links to the more active resource sites, taken from the list here (and checked myself). ...
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