Most will agree that Eurotubes is the best source for JJs since they only sell JJs! LOL. He actually sells full re-tube sets for different amps.
Here's a link to the F-series retube kits:
https://ssl.eurotubes.com/cart/index.php?page=view_products&category_id=6&sub_category_id=34
I would go with just the standard re-tube kit for the F-50. He'll make sure you receive a matched pair of power tubes within Mesa specs, three tested AX7s, and a balanced AX7 for your Phase Inverter slot. The preamp tubes do not need to be matched, only the power tubes. I think you will be blown away at how good these tubes sound in your amp! It will be quieter, tighter, cleaner, crisper, and crunchier! I know I was shocked when I hit the first chord in the clean channel after the swap. The distortion channel was much smoother and creamier too!