Bought an 870dm on sale for $200, thought it was a steal but I made the mistake of not buying extra magazine's while I was in the shop. Now I'm stuck with only 1 magazine and I haven't been able to find any places selling them other than ebay for $300+ each apparently.... Anybody know of any compatible magazines that another company sells?

well, patience and hope they make new ones some day in the future?  I cannot imagine wanting an extra mag enough to pay $300...  on the other hand, you could part out your gun and probably double your money.  Actually the mag and barrel alone should get you at least $400...  put the rest of the parts under the bed and see if the mags ever become available again.

https://www.reddit.com/r/870/comments/gsf3g6/is_there_anywhere_i_can_order_6rd_mags_for_the/

I bought one with the rebate for $200 as well. The standard Tac14 was $350, so I figured why not. Well no spare mags got me and is why not. I assumed that because Remington limited it to 6rd mags, somebody would make higher capacity mags & drums. Even if it was Promag, whose Saiga 12 drums seem run okay. I say that half joking as I have had horrible experience with Promag in the past but will forgive them if they can pull it out.  But I have sent a request to CS Spec as I really like their mags for my Vepr-12. Hopefully they can adapt the body of their mags without much trouble.

This is exactly why Remington went out of business. They don't sell the things we truly want. They limit the 870 DM to 6 rds, when Mossberg has 10 & 20rd for their version. This really is a range toy/mall ninja special for the most part, so give it the goods right from the begin. Have a MCS pattern hand guard and stock so people buy it, even if just to harvest the parts.

I bought it with the intention of doing a Form 4 and SBS it. A 6-8" barrel and folding stock would make for a nice compact package that still had decent capacity. I've always liked the super short Rem 870 conversions like the KEG-12 from Safety Harbor, but hated the 2-3 shell capacity. But I need mags for it before I sink any more money into it.

Where did you find an 870DM for $200???

They were $250 for the Tac-14 DM plus a $50 rebate from Remington a year ago.  I ordered mine online from a local gun shop in Chandler, AZ through their website.  They had some full size models for $275 (18") & $325 (24" camo version) before the rebate as well.  I almost bought a 18" model with ghost rings, but when with the Tac-14.  I believe the offer is long gone as a last ditch effort from Remington to drum up a last minute deal before they fell apart.