There are a few options but the main thing you want to make sure you look for is that you buy a conical burr grinder. You wouldn't want to use the $25 Krupps grinder you find in the coffee aisle at HEB. While that's fine for elevating a drip machine, it won't grind the coffee fine enough for an espresso extraction.
If you buy the Breville Barista that someone mentioned on this thread, it comes with a built-in grinder. I've never personally used it but I've heard good things. I paired a Breville Smart Grinder Pro with my Gaggia and I really liked the results I got. Sometimes you can get them on sale.
Another grinder I can personally recommend is the Baratza Encore or, if you can find one second-hand, the Baratza Sette 270. The only downside to the Encore is that it doesn't have a portafilter holder. Meaning, the grinds fall into a little can, and you have to pour the grinds into your portafilter basket.
If you got cash to burn and want to be big pimping, get the La Marzocco Lux D. Which is just an insanely beautiful machine.
A few other unsolicited tips...
- Considering we are talking about grinders I feel like this should go unsaid, but anyway...always buy whole beans, don't buy pre-ground stuff
- Buy a tamp. The tamps that come with grinders are plastic and dog sh*t. You can get one on amazon for cheap.
- De-scale your machine once a quarter (unless you're running purified RO water or whatever). It's easy, you can buy descaling material online and find YouTube videos. The better the machine the longer it'll last, but you also have to take care of it.
- YMMV, but I always prefer to dose my coffee. Meaning, that if I want a double shot I put 19 grams into the grinder because I know I get 19 grams out. Don't just dump your bag of beans into the grinder and expect to get consistent doses, unless you have a really serious grinder.
- Each bag of coffee will perform differently in the grind and extraction. Some beans grind easier than others (light vs dark). You'll sometimes hear about people "dialing in" a shot, and that's where they're pulling a couple of single shots at different grind settings to get the right balance and measuring contact time, extraction time, etc. You can nerd out as little or as much as you want.
- Espresso roasts tend to fall on the darker side of things, but don't shy away from a roaster who does a lighter roast on their espressos. I think you can get a ton more flavor the less you roast.
- Check roast dates on bags. Typically, an espresso roast needs about 10-14 days from roast date to settle in. So if you go to a roastery or a store and buy a bag of espresso roast and it has a roast date from 3-5 days ago, let it sit for a little while.
