Thumb shifters are preferable to the integrated bar levers and shifters. The cheapest by a long shot are the sunrace friction thumbshifters but the supply of these appears to have dried up during the pandemic. They are cheap and good if you can find a pair.

https://www.amazon.com/Sunrace-SLM10.../dp/B007QMJH86

You can also put up an ad in the WTB section of the C&V forum. If you look, there are some thumbies for sale there. But yeah get friction thumb shifters.

Velo orange sells these which will allow you to mount your downtube shifters on your handlebars but they are expensive:
https://velo-orange.com/products/vo-...unts-22-2-23-8

The 610 came stock with friction shifting. If you want to go indexing, shimano indexing thumb shifters are good but they'll have to match your freewheel (6 or 7). The old Deore shifters are very good and they have a friction option as well so they're a good choice when looking used.