Tommy Built T36. Tom Bostic's G36 clone. Available in 'pistol' or rifle configurations. He makes his own receiver, and uses US-made clone parts. All around a good rifle. Entry level: Ballpark $2700.
SL8 to G36 conversion. The 'traditional' route. Rifle config only. Rifle starts as an HK SL8. Tom Bostic modifies the magwell of the receiver, and installs a rear stub (or you can use a stock block). Price can vary widely, depending on options but $3500-4000 should get you one that has already been set up on the used market. Things to look for: Make sure the magwell has been correctly modified, and reinforced, rather than just left chopped up and open. Also note the rear of the receiver and whether it has be grafted with a G36 rear stub. Finally, there are the receiver re-markings to change it from saying "SL8" to "G36". All of these options represent a notch up in price, and then there are various OTHER options like, the particular top rail (dual optics can command $1500 or so all by themselves), and barrel assembly, etc.
G36 reweld. The holy grail of semi-auto G36s. Available in rifle or pistol configs, often as collaborations between Mike Otte and Tom Bostic. The weapon starts life as a real-deal, full-auto G36. The receiver is de-milled to ATF specs, and rebuilt as a semi-auto G36. It is the closest you can get to a factory G36 from H&K. Price is ballpark $5500 - $6000 when bought "off the shelf" from Mike Otte. Conceivably the price could be shaved down a smidge, if you were to arrange with Tom Bostic to take possession of a G36 machine gun to de-mill for you, and if you could find a post-sample G36 on the cheap.
If you have the funds, I would reach out to Michael's Machines (vendor here on the forum, just look around) and see what the availability on reweld G36's are. Failing that, I would have patience, and wait for a SL8 / G36 conversion to pop up -- one that has been done all the way, with the rear stub and remarked. Check the forum here, and Gunbroker. Post a want-to-buy when your post count is high enough.
I wouldn't hesitate to get a T36 clone either -- but I would definitely do that AFTER I got an HK-marked rifle. Lets face it. You don't want a clone. Heck, we don't EVEN want conversions or rewelds. We want factory G36's... but we take what we can get. That's me -- if you don't mind the idea of a clone, or if you're on a tight budget -- the T36 is a great option. At the end of the day, there is virtually NO WAY to get into a civvie G36 without Tom Bostic's involvement -- so grabbing a full clone from him and saving a few bucks isn't the worst idea one could have.
Keep us updated, and let us know what you end up getting!